[SOLVED] CS flex scheme Excel discrete mathematics algorithm AI chain ARTIFICIAL INTELLIGENCE

ARTIFICIAL INTELLIGENCE
1
The Structure-Mapping Engine : Algorithm and Examples
Brian Falkenhainer* and Kenneth D. Forbus
Qualitative Reasoning Group, Department of Computer Science, University of Illinois, 1304 West Springfield Avenue,
Urbana, IL 61801, USA
Dedre Gentner
Psychology Department, University of Illinois, Urbana, IL, USA
ABSTRACT
This paper describes the structure-mapping engine (SME), a program for studying . analogical processing.SME has been built to explore Gentners structure-mapping theory of analogy, and provides a tool kit for constructing matching algorithms consistent with this theory. Its flexibility enhances cognitive simulation studies by simplifying experimentation. Furthermore, SME is very efficient, making it a useful component in machine learning systems as well. We review the structure-mapping theory and describe the design of the engine. We analyze the complexity of the algorithm, and demonstrate that most of the steps are polynomial.typically bounded by O(N) .Next we demonstrate some examples of its operation taken from our cognitive simulation studies and work in machine learning. Finally, we compare SME to other analogy programs anddiscuss several areas for future work.
1. Introduction
In analogy, a given situation is understood by comparison with another similar situation. Analogy may be used to guide reasoning, to generate conjectures about an unfamiliar domain, or to generalize several experiences into an abstract schema. Consequently, analogy is of great interest to both cognitive psychologists and artificial intelligence researchers. Psychologists aim to clarify the mechanisms underlying analogy in order to understand human learning and reasoning. Artificial intelligence researchers aim to emulate analogical proces- sing on computers to produce more flexible reasoning and learning systems .
This paper describes the structure-mapping engine (SME), a program built to * Current address: Xerox Palo Alto Research Center. 3333 Coyote Hill Road. Palo Alto, CA
94304, USA
(1004-3702/89/S3 .50 1989, Elsevier Science Publishers BY (North-Holland)
Artificial Intelligence 41 (1989/90) 1-63

2 B. FALKENHAINER ET AL.
explore the computational aspects of Gentners structure-mapping theory of analogical processing [28.30].SME constructs all consistent ways to interpret a potential analogy and does so without backtracking . It is both flexible and efficient. Beyond this, SME provides a -tool kit for building matchers that satisfy the structural consistency constraint of Gentners theory . Additional constraints defining a matcher are specified by a collection of rules. which indicate local, partial matches and estimate how strongly they should be believed. The program uses these estimates and a novel procedure for combin- ing the local matches to efficiently produce and evaluate all consistent global matches .
Cognitive simulation studies can offer important insights for understanding the human mind . They serve to verify psychological theories and supply a
detailed vocabulary for describing cognitive processes. Cognitive simulations can provide idealized subjects, whose prior knowledge and set of available processes is completely known to the experimenter. Unfortunately, cognitive simulations tend to be complex and computationally expensive (cf. [2, 72]). Complexity can obscure the relationship between the theory and the program . While all design decisions affect a programs performance, not all of them are directly motivated by the theory being tested. To assign credit properly (or to model performance in detail) requires exploring a space of similar architec- tures. Suchexplorations are very difficult if the major way to change the programs operation is surgery on the code. Complex programs also tend to be computationally expensive, which usually means fewer experiments are per- formed and fewer possibilities are explored. While there have been several important Al programs that study computational aspects of analogy (e.g., [5, 78, 79]), they were not designed to satisfy the above criteria.,
Over the last decade there have been a variety of programs that simulate different aspects of analogical processing (as reviewed in Section 6). However, the progress to date has been disappointingly slow . Often papers describe programs that work on only a handful of carefully chosen examples, and do not specify the algorithms in a replicable fashion. We believe the difficulty has been due in part to the lack of a good problem decomposition . Without some theoretically motivated decomposition of analogy, it is easy to conflate distinct problems, and become lost in the space of possible mechanisms . Our decompo- sition, described in the next section, is psychologically motivated . Roughly, SME focuses on the mapping process in analogy, leaving the access and application aspects to future studies. The power of the program and its success on a wide variety of examples (over 40 as of this writing) provides additional evidence that the decomposition is a good one.
This paper examines the architecture of the structure-mapping engine and how it has been used for machine learning and cognitive simulation . First, we review Gentners structure-mapping theory and some of the psychological evidence for it. Next we discuss the organization of SME, including knowledge representation conventions and the algorithm . After a complexity analysis. we

THE STRUCTURE-MAPPING ENGINE 3
then illustrate SMEs operation on several examples drawn from machine learning and cognitive simulation studies. Related work in both Al and psychology is reviewed next, followed by a discussion of future work .
2. Structure-Mapping Theory
The theoretical framework for this research is Gentners structure-mapping theory of analogy [28-32.34]. Structure-mapping describes the set of implicit constraints by which people interpret analogy and similarity . The central idea is that an analogy is a mapping of knowledge from one domain (the base) into another (the target) which conveys that a system of relations known to hold in the base also holds in the target. The target objects do not have to resemble their corresponding base objects. Objects are placed in correspondence by virtue of corresponding roles in the common relational structure .
This structural view of analogy is based on the intuition that analogies are about relations, rather than simple features. No matter what kind of knowl- edge (causal models, plans, stories, etc.), it is the structural properties (i.e.,the interrelationships between the facts) that determine the content of an analogy. For example, consider the water flow and heat flow situations shown in Fig . 1 . These situations are thought to be analogous because they share the complex relationship known as flow. In each, we have a notion of something flowing downhill, from a source to a destination . We prefer to ignore the appearances and even specific defining properties of the objects, such as the fact that water and coffee are both liquids. Indeed, focusing on these attributes tends to confuse our picture of the analogy.
2.1. Subprocesses in analogy
Structure-mapping decomposes analogical processing into three stages ([27, 31, 35], see also [10, 11, 42, 51]):
(1) Access: Given a current target situation, retrieve from long-term me- mory another description, the base, which is analogous or similar to the target .
Fig. 1. Two physical situations involving flow (adapted from [zl) .

4 B. FALKENHAINER ET AL .
(2) Mapping and inference:Construct a mapping consisting of correspond- ences between the base and target . This mapping can include the candidate inferences sanctioned by the analogy, which specify what additional knowledge in the base can potentially be transferred to the target .
(3) Evaluation and use : Estimate the quality of the match . Three kinds of criteria are involved [31 . 321. The structural criteria include the number of similarities and differences, the degree of structural similarity involved, and the amount and type of new knowledge the analogy provides via the candidate inferences. The second criterion concerns the validity of the match and the inferences it sanctions. The inferences must be checked against current world knowledge to ensure that the analogy at least makes sense, and may require additional inferential work to refine the results. The third criterion is relevance, i.e., whether or not the analogy is useful to the reasoners current purposes . Structure-mapping focuses on structural criteria only, since they define and distinguish analogy from other kinds of inference .
The structure-mapping engine emulates the mapping stage of analogy and provides a structural, domain-independent evaluation of the match . In other work we have used SME in modeling access and evaluation .(17,18b,68u1 t here we focus on mapping and structural evaluation .
2.2. Constraints on analogy
Structure-mapping defines similarity in terms of matches between the internal structures of the descriptions being compared. Consequently, we need some terminology for describing such structures . Section 3.1 will introduce several formal descriptions. Here we provide some motivating intuitions. Consider a propositional statement, like
CAUSE[GREATER-THAN(x, y), BREAK(x)J
The chief relation involved in this statement is CAUSE, and its arguments are GREATER-THAN(x, y) and BREAK(x) . We can view this statement in the usuful
way as a tree, i.e., the root of the tree is a node whose label is the predicate CAUSE and the roots children are nodes representing the relations arguments
(Fig. 2 provides an example of this view). This view is useful in understanding structure-mapping because it provides a spatial metaphor for collections of statements. For instance, we can say that the arguments are below the CAUSE statement in the internal structure of the description, and describe a collection of statements with logical constraints between them (explicitly represented by statements involving logical connectives and/or relationships) as a connected system of relations.
One formal definition is needed before proceeding. We define the order of an item in a representation as follows: Objects and constants are order 0. The

i
THE STRUCTURE-MAPPING ENGINE
WATER-FLOW
CAUSE
W(
GREATER FLOW (beaker, vial. water.pixel
PCIESSURE(beakerl PRESSURE(vial)
5
UOWD(waterl FLAT-TOP (waterI
GREATER
DIAMETER (beaker) DIAMETER (vial)
HEAT-FLOW
GRF~TER
TEMPERATURE (Coffee) TEMPERATURE ;iCe-cuoel
FLOW (coffee . c*-cube . meat. Dar)
LIOUIO(coffeel FLAT-TOP(coffee)
Fig. 2. Simplified water flow and heat flow descriptions,.
order of a predicate is one plus the maximum of the order of its arguments . Thus GREATER-THAN(x, y) is first-order if x and y are objects, and CAUSE (GREATER-THAN(x, y), BREAK(x)] is second-order. Examples of higher-order relations include CAUSE and IMPLIES . This definition of order should not be confused with the standard definition of the order of a logic . Using the tree view of statements, this definition of order indicates how deep the structure is below an item. Notice that intricate explanations with many layers of justifica- tions can give rise to representation structures of higher order, since there will be a high degree of nesting.
Let {B,), (T;) denote the items in the base and target representations, respectively. Let the subsets (b,), {t;} denote the objects in the base and target, respectively. The tacit constraints on the analogical mapping M can be charac- terized as follows:
(1) Objects in the base are placed in correspondence with objects in the target:
M :b;t;
(2) Isolated object descriptions are discarded unless they are involved in a larger relational structure:
M :RED(b;)-4RED(t,) .
(3) Relations between objects in the base tend to be mapped across :
M :COLLIDE(b;,b,)COLLIDE(t,, t,)
(4) The particular relations mapped are determined by systematicity, as
Under the standard definition, a logic is first-order if variables only range over objects and second-order when it permits variables to range over predicates as well .

6 B. FALKENHAINER ET AL.
defined by the existence of higher-order constraining relations which can themselves be mapped:
M :CAUSE(PUSH(b,, b,),COLLIDE(b,, b.,)] CAUSE(PUSH(t,, t,), COLLIDE(t,, tk)]
We require M to be one-to-one:that is. no base item maps to two target items and no target item maps to two base items . Furthermore, we require M
to be structurally consistent . This means that, in addition to being 1 : 1. if M maps B, onto T,, then it must also map the arguments of B, onto the corresponding arguments of T, .
Consider for example a simple analogy between heat flow and water flow . Figure 2 shows a simplified version of what a learner might know about the
situations pictured in Fig . 1. In order to comprehend the analogy heat is like water a learner must do the following (although not necessarily in this order):
(1) Set up the object correspondences between the two domains :
water- heat, pipe bar, beaker -+ coffee, vial- ice-cube
(2) Discard object attributes, such as LIOUID(water) . (3) Map base relations such as
_ GREATER-THAN(PRESSURE(beaker), PRESSURE(vial)J
to the corresponding relations in the target domain .
(4) Observe systematicity. i.e.. keep relations belonging to a systematic
relational structure in preference to isolated relationships. In this example.
CAUSE(GREATER-THAN(PRESSURE(beaker), PRESSURE(vial)J, FLO1M(beaker, vial, water, pipe))
is mapped into
CAUSE(GREATER-THAN(TEMPERATURE(coffee), TEMPERATURE (ice-cube)),
FLOW(coffee, ice-cube, heat, bar))
while isolated relations, such as GREATER-THAN(DIAMETER(beaker), DIAMETER(vial)J
are discarded.
I

THE STRUCTURE-MAPPING ENGINE 7
The systematicityprinciple is central to analogy. Analogy conveys a system of connected knowledge. not a mere assortment of independent facts. Preferring systems of predicates that contain higher-order relations with inferential import is a structural expression of this tacit preference for coherence and deductive power in analogy. Thus, it is the amount of common higher-order relational structure that determines which of several possible matches is preferred . For example. suppose in the previous example we were concerned with objects differing in specific heat . such as a metal ball-bearing and a marble of equal mass, rather than temperatures. Then DIAMETER would enter the mapping instead of (or in addition to) PRESSURE. since DIAMETER affects the capacity of a container, the analogue to specific heat .
2.3. Other types of similarity
In addition to analogy, the distinctions introduced by structure-mapping theory provide definitions for several other kinds of similarity . In all cases, we require
one-to-one. structurally consistent mappings. As we have seen, in analogy only relational structures are mapped. Aspects of object descriptions which play no
role in the relational structure are ignored. By contrast, in literal similarity both relational predicates and object descriptions are mapped . Literal similarity typically occurs in within-domain comparisons . in which the objects involved look alike as well as act alike. An example of a literal similarity is the comparison Kool-Aid is like juice. In mere-appearance matches, it is pri- marily the object descriptions which are mapped, as in the metaphor
The road is like a silver ribbon.
A fourth kind of mapping is the abstraction mapping.Here, the entities in the base domain are variables, rather than objects . Few, if any, attributes exist that do not contribute to the bases relational structure . Applying an abstrac- tion match is very close to the instantiation of a rule . The difference is that only entities may be variables, whereas in many pattern-directed rule systems predicates may be used in substitutions as well .
2.4. Empirical evidence
Although the focus of this paper is on computational modeling, two sets of psychological findings are particularly relevant. First, empirical psychological studies have borne out the prediction that systematicity is a key element of peoples implicit rules for analogical mapping. Adults focus on shared sys- tematic relation structure in interpreting analogy . They tend to include rela-
Notice that our structural characterization of literal similarity differs from some other psycho- logical approaches (e.g..1701).

8
B. FALKENHAINER ET AL.
tions and omit attributes in their interpretations of analogy, and they judge analogies as more sound and more apt if base and target share systematic
relational structure [9 . 28, 33, 35, 361. In developmental work, it has been found that eight-year olds (but not five-year olds) are better at performing difficult mappings when the base structure is systematic [371 . Second . there is also empirical evidence that the different types of similarity comparisons defined by structure-mapping have different psychological properties [30-32] .
3. The Structure-Mapping Engine
A simulation of Gentners theory has been implemented in the structure- mapping engine (SME) .Given descriptions of a base and target, SME constructs all structurally consistent mappings between them . The mappings consist of pairwise matches between statements and entities in the base and target, plus the set of analogical inferences sanctioned by the mapping.SME also provides a structural evaluation score for each mapping according to the constraints of systematicity and structural consistency . For example, given the descriptions of water flow and heat flow shown in Fig. 2, SME would offer several alternative interpretations. In one interpretation, the central inference is that water flowing from the beaker to the vial corresponds to heat flowing from the coffee to the ice cube. Alternatively, one could map water to coffee, since they are both liquids.The first interpretation has a higher structural evaluation score than the second, since a larger relational structure can be mapped.
Importantly, SME is not a single matcher, but a simulator for a class of matchers. The structure-mapping notion of structural consistency is built into the system. However, which local elements can match and how these combina- tions are scored can be changed by implementing new match rules that govern
what pairwise matches between predicates are allowable and provide local measures of evidence. Thus, for example, SME can be used to simulate all the similarity comparisons sanctioned by structure-mapping theory, not just ana- logy. Since the match rules can include arbitrary LISP code, it is possible to implement many other kinds of matchers as well .
This section describes the SME algorithm in sufficient detail to allow replica- tion. We start by specifying some simple conventions for knowledge repre-
sentation which are essential to understanding the algorithm.
3.1. Representation conventions
We make as few representational assumptions as possible so that SME remains domain-independent. We use a typed (higher-order, in the standard sense) predicate calculus to represent facts. The constructs of this language are :
Entities:Individuals and constants.
Predicat.s:There are three types :functions. attributes.and relations. Each
is described below.
1

THE STRUCTURE-MAPPING ENGINE 9
Dgroup : A description group is a collection of entities and facts about them. considered as a unit.
We examine each construct in turn.
3.1.1. Entities
Entities are logical individuals, i.e., the objects and constants of a domain. Typical entities include physical objects, their temperature, and the substance
they are made of. Primitive entities are the tokens or constants of a description and are declared with the defEntity form:
(defEntity (name) [:type (EntityType) ] [:constant? {tInil}])
Entities can also be specified in the usual way by compound terms, i .e. the term (pressure We1132) refers to a quantity.
The :type option establishes a hierarchy of entity types . For example. we state that our sun is a particular instance of a star with
(defEntity sun :type Star)
Constants are declared by using the :constant? option, as in
(defEntity zero :type number :constant? t)
3.1.2. Predicates
Classically. predicate refers to any functor in a predicate calculus statement. We divide predicates into three categories:functions, attributes, and relations. Each is treated differently under structure-mapping.
Functions : Functions map one or more entities into another entity or constant. For example, (PRESSURE piston) maps the physical object piston into the quantity which describes its pressure . We treat boolean predicates as attributes or relations (see below), rather than functions. Structure-mapping
allows substitution of functions to acknowledge their role as an indirect way of referring to entities. All other predicates must be matched identically.
Attributes:An attribute describes some property of an entity. Examples of attributes include RED and CIRCLE. We restrict attributes to take only one
argument-if there are multiple arguments we classify the predicate as a relation. It is well-known that a combination of a function and a constant can be logically equivalent to an attribute. For example,
(RED BlockA)

10
and
(= (COLOR BlockA) RED)
B. FALKENHAINER ET AL .
are logically equivalent. However, these two forms do not behave identically under structure-mapping. In analogy, attributes are ignored unless they are part of some higher-order structure. When attributes are matched (e.g..literal similarity and mere-appearance comparisons), they must match identically . We assume that a reasoner has a particular piece of information represented in one form or another, but not both, at any particular time (we return to this issue in
Section 7.1).
Relations: Like attributes, relations range over truth values . Relations
always have multiple arguments, and the arguments can be other predicates as well as entities. (However, we classify logical connectives. regardless of the number of arguments, as relations .) Examples of relations include CAUSE, GREATER-THAN, and IMPLIES. In structure-mapping. relations must always match identically.
Predicates are declared with the defPredicate form. It has several options:
(defPredicate (Name) (ArgumentDeclarations) (PredicateType ) :expression-type (Defined Type)
(:commutative? (tInil)]
[:n-ary? (tinil)])
(Predicate Type) is either function, attribute, or relation, according to what kind of predicate (Name) is. The (ArgumentDeclarations) specifies the predicates arity and allows the arguments to be named and typed. For example, the declaration:
(defPredicate CAUSE ((antecedent sevent) (consequent sevent)) relation)
states that CAUSE is a two-place relational predicate. Its arguments are called antecedent and consequent, both of type sevent. (We use sevent to mean the union of states and events.) The names and types of arguments are for the
convenience of the representation builder, and are not currently used by SME . However, the predicate type is very important to the algorithm, as we will see
below.
The optional declarations :commutative? and :n-ary? provide SME with im-
portant syntactic information. :commutative? indicates that the predicate is commutative, and thus the order of arguments is unimportant when matching . :n-ary? indicates that the predicate can take any number of arguments. Declar-
ing n-ary predicates reduces the need for applying associativity to binary predicates 1691, allowint;SME to avoid explicitly encoding associative laws in

THE STRUCTURE-MAPPING ENGINE
11
the matcher. Examples of commutative n-ary predicates include AND. OR, and SUM .
3.1.3. Expressions and dgroups
For simplicity, predicate instances and compound terms are called expressions . A description group, or dgroup, is a collection of primitive entities and
expressions concerning them . Dgroups are defined with the defDescription form :
(defDescription (DescriptionName) entities((Entity,), (Entity,) (;En)ti)ty expressions ((Expression Declarations) ))
where (Expression Declarations) take the form
(expression) or ((expression):name(ExpressionName))
The :name option is provided for convenience;(expression) will be substituted for every occurrence of (Expression Name) in the dgroups expressions when the dgroup is created. For example, the description of water flow depicted in Fig. 2 was given to SME as
(defDescription simple-water-flow
entities (water beaker vial pipe)
expressions (((flow beaker vial water pipe) :name wflow)
((pressure beaker) :name pressure-beaker) ((pressure vial) :name pressure-vial) .
((greater pressure-beaker pressure-vial) :name >pressure) ((greater (diameter beaker) (diameter vial))
:name >diameter)
((cause >pressure wflow) :name cause-flow) (flat-top water)
(liquid water)))
The description of heat flow depicted in Fig. 2 was given to SME as
(defDescription simple-heat-flow
entities (coffee ice-cube bar heat)
expressions (((flow coffee ice-cube heat bar) :name hflow)
((temperature coffee) :name temp-coffee)
((temperature ice-cube) :name temp-ice-cube)
((greater temp-coffee temp-ice-cube) :name >temperature) (flat-top coffee)
(liquid coffee)))

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B. FALKENHAINER ET AL .
Notice that each expression does not need to be declared explicitly: for example.SME will automatically create and name expressions corresponding to (diameter beaker) and (diameter vial) in the water flow description .
We will refer to the expressions and entities in a dgroup collectively as items. To describe the SME algorithm we need some terminology to express the structural relations between items . These relationships form directed acyclic graphs. so we adopt some standard graph-theory terminology . Each item
corresponds to a vertex in a graph. When item 5, has 5, as an argument . there will be a directed arc from the node corresponding to 5, to the node
corresponding to 5,. The offspring of an expression are its arguments. By definition, primitive entities (i.e.. those denoted by constants) have no off- spring. Expressions which name entities by compound terms are treated like any other item. An item 5, which is in the transitive closure (arguments of arguments, etc.) of another item 5, is said to be a descendant of 5,, while 5, is said to be an ancestor of J,. An item with no ancestors is called a root. The term Reachable(.) refers to the transitive closure of the subgraph starting at J . We define the depth of an item with respect to Reachable(,) by the minimum .number of arcs it takes to reach the item starting from J.
3.2.The SME algorithm: Overview
Given descriptions of a base and a target, represented as dgroups, SME builds all structurally consistent interpretations of the comparison between them. Each interpretation of the match is called a global mapping, or gmap .3Gmaps consist of three parts:
(1) Correspondences : A set of pairwise matches between the expressions and entities of the two dgroups.
(2) Candidate inferences:A set of new expressions which the comparison suggests holds in the target dgroup.
(3) Structural evaluation score (called SES for brevity):A numerical estimate of match quality based on the gmaps structural properties.
Following the structure-mapping theory, we use only purely structural criteria to construct and evaluate the mapping.SME has no other knowledge of either base or target domain. Neither rules of inference nor even logical connectives themselves are built into the algorithm . Each candidate inference must be interpreted as a surmise, rather than a logically valid conclusion . The SES reflects the aesthetics of the particular type of comparison, not validity or
The definition of gmap is inspired in part by de Kleei s work on assumption-hosed truth maintenance, although we do not use an ATMS in the actual code . The idea of combining local solutions by constructing maximally consistent sets is analogous to the process of interpretation construction in an ATMS. We also find hit-vectors a useful implementation technique for the set of operations needed to maintain structural consistency .

THE STRUCTURE-MAPPING ENGINE 13
potential usefulness. Testing the validity of candidate inferences and determin- ing the utility of a match are left to other modules, as described in Section 2 .
Match rules specify which pairwise matches are possible and provide local measures of quality used in computing the SES. These rules are the key to
SMEs flexibility. To build a new matcher one simply loads a new set of match rules. This has several important advantages . First, we can simulate all of the
similarity comparisons sanctioned by structure-mapping theory with one pro- gram. Second. we could in theory*tune- the rules if needed to simulate particular kinds of human performance (although, importantly . this flexibility has not been needed so far!) . Third, we can also simulate a number of other analogy systems (including [43, 781, as described below) for comparison pur- poses.
Conceptually, the SME algorithm is divided into four stages:
(1) Local match construction :Finds all pairs of ((Baseltem), (Targetltem)) that can potentially match.A match hypothesis iscreated for each such pair to
represent the possibility that this local match is part of a global match.
(2) Gmap construction:Combines the local matches into maximal consistent
collections of correspondences.
(3) Candidate inference construction : Derives the inferences suggested by
each gmap.
(4) Match evaluation:Attaches evidence to each local match hypothesis and
uses this evidence to compute structural evaluation scores for each gmap.
We now describe each computation in detail, using a simple example to illustrate their operation.
3.2.1. Local match construction (Step 1)
Given two dgroups, SME begins by finding potential matches between items in the base and target (see Fig . 3). Allowable matches are specified by match
constructor rules, which take the form :
(MHCrule ((Trigger) (BaseVariable) (TargetVariable ) [:test (TestForin)])
(Body) )
In all match constructor rules, (Body) will be executed in an environment in which (BaseVariable) and (TargerVariable) are bound to items from the base and target dgroups, respectively. If (TestForm) is present. the bindings must satisfy the test (i.e., the form when evaluated must return non-NIL) . There are two possible values for (Trigger) . A filter trigger indicates that the rule is applied to each pair of items from the base and target . These rules create an initial set of match hypotheses between individual base and target expressions . For example, the following rule hypothesizes a match between any two

1.1
B. FALKENHAINER ET AL.
13 MH between predicates
A kH between entities (Emap)
Fig . 3. Local match construction . The graphs corresponding to the water flow and heat flow descriptions of Fig. 2 are depicted on the left and right panels, respectively . The squares and triangles in the middle represent the match hypotheses created by the literal similarity rules for these dgroups. The dashed arrows indicate which base and target items are conjectured as matching by each match hypothesis. The squares represent match hypotheses involving expres- sions, while the triangles represent match hypotheses involving entities . Notice how sparse the match is. Expression matches are only created when relations are identical . and matches between functions and entities are only created to support expression matches. This middle out local match computation provides SME with much of its power .
expressions that have the same functor:
(MHCrule (filter ?b ?t :test (equal (expression-functor ?b)
(install-MH ?b ?t))
(expression-functor ?t)))
An :intem trigger indicates that the rule should be run on each newly created match hypothesis, binding the variables to its base and target items . These rules create additional matches suggested by the given match hypothesis . For example, hypothesizing matches between every pair of entities would lead to combinatorial explosions. Instead, we can use an antem rule to create match hypotheses between entities in corresponding argument positions of other match hypotheses, since these correspondences will be required for structural consistency.
Appendix A lists the rule sets used to implement each similarity comparison of structure-mapping (analogy, literal similarity, and mere appearance).Notice
that each rule set is small and simple (we describe the evidence rules below). The literal similarity rule set uses only three match constructor rules. One rule

THE STRUCTURE-S,MAPPI`G ENGINE 15
is the filter rule shown above . The other two are intern rules . The content of the first is, roughly.
If the match hypothesis concerns two expressions . then create match hypotheses between any corresponding arguments that are
both functions or entities.
The second is a specialization of this which runs only on commutative predicates (i.e.. the corresponding arguments condition is removed) . The analogy rule set differs in that matches are created between attributes only when they are part of some higher-order structure . The mere-appearance rule set differs by completely ignoring higher-order structure .
The result of running the match constructor rules is a collection of match
hypotheses.Wedenotethehypothesisthatb;andt,matchbyMH(b;,tj). When no ambiguity will result, we will simply say M H . We will use the same
terminology to refer to the structural properties of graphs of match hypotheses (offspring, descendants. ancestors, root) as we use for describing items in
dgroups. As with dgroups, the collection of match hypotheses can be viewed as a directed acyclic graph, with at least one (and possibly many) roots .
Example (Simple analogy between heat and water).In this example we will use
the literal similarity rule set, rather than analogy, in order to better illustrate the algorithm. The result of running these rules on the water flow and heat flow dgroups of Fig. 2 is shown in Fig. 3 (see also Fig. 4). Each match hypothesis locally pairs an item from the base dgroup with an item from the target dgroup .
There are several points to notice in Fig . 4. First, there can be more than one match hypothesis involving any particular base or target item. Here. TEMPERATURE can match with both PRESSURE and DIAMETER, since there are corresponding matches between the GREATER-THAN expressions in both dgroups (MH-1 and MH-6) . Second, note that all predicates which are not functions must match identically. Entities, on the other hand, are matched on the basis of their roles in the predicate structure . Thus while TEMPERATURE can match either PRESSURE or DIAMETER, GREATER cannot match anything but GREATER. This distinction reflects the fact that functions are often used to refer to objects or constants, which are fair game for substitution under analogy. Third, not every possible correspondence is created. We do not, for example, attempt to match TEMPERATURE with water or heat with beaker. Functions only match with other functions: and local matches between entities are only created when justified by some other match . In general, this signifi- cantly constrains the number of possible matches.
3.2.2. Global match construction (Step 2)
The second step in the SME algorithm combines local match hypotheses into collections of global matches (gmaps). Intuitively, each global match is the

16
B . FALKENHAINER ET AL .
MH-2
MH-3
MH-7 MH-e
8: Diameter-1 B: Diameter-2
T: Temp-coffee T: Temp-ice-cube
MH-12
B: Flat-top-4 T: Flat-top-6
MH-14 8: water T: coffee
MH-1
B: >Pressure
T: >Temperature
MH-9
B: Wflow T : Hfiow
B: Pressure-beaker 8: Pressure-vial T: Temp-coffee T: Temp-ice-cube
MH-4
6 : beaker T: coffee
M +-S
8 : vial
T: ice-cube
MH-13 B:Liquid-3 T: Liquid-5
Fig. 4. Water flow/heat flow analogy after local match construction . Here we show the graph of match hypotheses depicted schematically in Fig. 3. augmented by links indicating expression-to- arguments relationships. Match hypotheses which are not descended from others are called roofs (e.g..the matches between the GREATER predicates.MH.1 and MH-6. and the match for the predicate FLOW. MH-9) . Match hypotheses between entities are called emaps (e.g.. the match between beaker and coffee, MH-4). Emaps play an important role in algorithms based on structural
consistency.
largest possible set of match hypotheses that depend on the same one-to-one object correspondences.
More formally, gmaps consist of maximal, structurally consistent collections of match hypotheses . A collection of match hypotheses is structurally consistent
if it satisfies two constraints:
(1) One-to-oneness:The match hypotheses in the collection do not assign the same base item to multiple target items or any target item to multiple base items.
(2) Support : If a match hypothesis MH is in the collection, then so are match hypotheses which pair the arguments of MHs base and target items.
The one-to-one constraint allows straightforward substitutions in candidate inferences. The support constraint preserves connected predicate structure . A collection is maximal if adding any additional match hypothesis would render the collection structurally inconsistent .
Requiring structural consistency both reduces the numoer of possible global
MH-10 MH-11
8: water T: heat
B: pipe T: bar
MH-6
B: >Olameter
T: >Temperature

THE STRUCTURE-MAPPING ENGINE 17
collections and helps preserve the soundness and plausibility of the candidate inferences. Without it, every collection of local matches would need to be
considered, and effort would be wasted on degenerate many-to-one mappings without any possible inferential value . The maximality condition also serves to reduce the number of gmaps, since otherwise every subset of a gmap could itself be a gmap.
Global matches are built in two steps:
(1) Compute consistency relationships : For each match hypothesis . generate (a) the set of entity mappings it entails, (b) which match hypotheses it locally conflicts with, and (c) which match hypotheses it is structurally inconsistent
with.
(2) Merge match hypotheses : Compute gmaps by successively combining
match hypotheses as follows:
(a) Form initial combinations : Combine the descendents of the highest-
order structurally consistent match hypotheses into an initial set of
gmaps .
(b) Combine dependent gmaps :Merge initial gmaps that have overlapping
base structure, subject to structural consistency.
(c) Combine independent collections:Form complete gmaps by merging
the partial gmaps from the previous step, subject to structural con- sistency, keeping only the maximal results.
Importantly. the process of gmap construction is independent of gmap evaluation. Which gmaps are constructed depends solely on structural con- sistency. Numerical evidence, described below, is used only to compare their relative merits.
We now describe the algorithm in detail.
Computing consistency relationships
Consistency checking is the crux of gmap construction . To doo this we compute for each match hypothesis (a) the entity mappings it entails and (b) the set of match hypotheses it is inconsistent with.
Consider a particular match hypothesis MH(b ;,t,)involving base item b, and target item t. . If b ., t, are expressions, then by the support constraint the match
hypotheses linking their arguments must also be in any collection that MH(b;,ti) isin.Applyingthisconstraintrecursively,alldescendantsof
MH(b ;,t.)must be in the same collection ifit is structurally consistent (see Fig. 5). Since the chain of descendants ends with match hypotheses involving entities, each match hypothesis implies a specific set of entity correspondences :
Definition 3.1. An emap is a match hypothesis between entities. Emaps(MH(b ;,t,)) represents the set of emaps implied by a match hypothesis MH(b,, ti). Emaps(MH(b ;,t,)) is simply the union of the cmaps supported by

18
Water Flow
B. FALKENHAINER ET AL.
o -MH between predicates
MH between enuties (Emap)
Fig. 5. Water flow/heat flow analogy after computation of Conflicting relationships. Simple lines show the tree-like graph that the support constraint imposes upon match hypotheses. Lines with
circular endpoints indicate the Conflicring relationships between matches. Some of the original lines from match hypothesis construction have been left into show the source of a few Conflicting relations.
MH(b;,t/)sdescendants. We also include match hypotheses involving func- tionsinEmaps(MH(b;,t/)).
To enforce one-to-one mappings we must associate with each MH(b,.t;)the set of match hypotheses that provide alternate mappings for b;and ti.Clearly, no member of this set can be in the same gmap with MH(b;.ti).
Definition 3.2. Given a match hypothesis MH(b,, t,), the set Conflicting (MH(bi,ti))consistsofthesetofmatchhypothesesthatrepresentthealter- nate mappings for b, and t,:
Conflicting(MH(b ;. ti))
M [ U {MH(bkI ti)l bk O b,}] U [ U {MH(bi .rk)I tk O t /}] . hkEbase r,tErnret
The set Conficting(MH(b;.t,)only notes local inconsistencies (see Fig . 5). However,wecanuseitandEmaps(MH(b;,ti))torecursivelydefinethesetof
all match hypotheses that can never be in the same gmap as M H ( b i,ti).
Definition 3.3. The set NoGood(MH;)is the set of all match hypotheses which can never appear in the same gmap as MH;.This set is recursively defined as follows:ifMH;isanemap.thenNoGood(MH;) Conflicting(MH;).Other-

THE STRUCTURE-MAPPING ENGINE
19
wise, NoGood(MH,) is the union of MH,s Conflicting set with the NoGood sets for all of its descendants, i.e.,
NoGood(MH,) = Conflicting(MH,) U U NoGood(MH,) . l1H,EAr,s(.NH,)
We compute Conflicting, Ernaps, and NoGood sets as follows. First.Con- flicting is computed for each match hypothesis, since it requires only local
information. Second.Emaps and NoGood are computed for each emap. Third. Emaps and NoGood sets are computed for all other- match hypotheses by
propagating the results from Emaps upwards to their ancestors .
We make two observations about this computation . First, these operations can be efficiently implemented via bit vectors. For example, SME assigns a
unique bit position to each match hypothesis, and carries out union and intersection operations by using OR and AND bit operations. Second, it is important to look for justification holes in the match hypothesis graph-match hypotheses whose arguments fail to match . Such match hypotheses will always violate the support constraint, and hence should be removed . For example, if one of the PRESSURE-TEMPERATURE match hypotheses had not been formed (see Fig. 4), then the match between their governing GREATER predicates would be removed. Notice that removing justification holes eliminates many blatantly incorrect matches, such as trying to place an eighth-order IMPLIES in correspondence with a second-order IMPLIES.
The next step in gmap construction is to identify those match hypotheses which are internally inconsistent, and thus cannot be part of any gmap. This can happen when the descendants of a match hypothesis imply mutually incompatible bindings.
Defintion 3.4. A match hypothesis is inconsistent if the emaps entailed by one subgraph of its descendants conflicts with the emaps entailed by another
subgraph of its descendants:
Inconsistent(MH;) p Emaps(MH;)n NoGood( MH;)$ 0 .
Clearly, every ancestor of an inconsistent match hypothesis is also inconsistent . By caching the NoGood sets, inconsistent match hypotheses can be identified
easily.
Global match construction proceeds by collecting sets of consistent match hypotheses. Since gmaps are defined to be maximal, we begin from roots and
work downward rather than starting bottom-up.If a root is consistent, then the entire structure under it must be consistent . and thus forms an initial .gmap . If the graph of match hypotheses had only a single consistent root, this step

20 B. FALKENHAINER ET AL .
would suffice. However, typically there are several roots, and hence several initial gmaps. To obtain true gmaps, that is. maximal collections of match
hypotheses, these initial gmaps must then be merged into larger, structurally consistent collections.
Merge step 1: Form initial combinations. The first step is to combine interconnected and consistent structures (Fig .6(a)). Each consistent root, and its descendants, forms an initial gmap. If a root is inconsistent, then the same procedure is applied recursively to each descendant (i.e.. each immediate descendant is now considered as a root) . The resulting set will be called Gmaps, .The procedure is:
(1) Let Gmaps, = 0. (2).For every root MH(b i,t)
(a)if-ilnconsistent(MH(b;,ti)),thencreateagmapGM such thatElements(GM) = Reachable(MH(b,.t,));
(b) if lnconsistent(MH(bi,t)), then recurse on Offspring
;.)) (MH(b,, t
(3) For every GM E Gmaps,
(a) NoGood(GM) = U
(b) Emaps(GM) =
NoGood(MH(b,, td);
U Emaps(MH(b;.,ti)). .NH(bj.tj)E Roots(GM )
MH( bj.tj)ERoots(GM)
In this step inconsistent match hypotheses have been completely eliminated. However, we do not yet have true gmaps, since the sets of correspondences are not maximal. To obtain maximality, elements of Gmaps, that are consistent
(a) (b) (c)
7 MH betwan predieata
A MH betweea enuua IEmap(
Fig. 6. Gmap construction. (a) Merge step 1: Interconnected and consistent. (b) Merge step 2: Consistent members of the same base structure. (c) Merge step 3: Any further consistent
combinations.
7

THE STRUCTURE-MAPPING ENGINE 21
with one another must be merged . Consistency between two gmaps can be defined as follows:
Consistent(GMap;, GMap,)
iff Elements(GMap,) nNoGood(GMap,) = 0
A NoGood(GMap;)nElements(GMap,) = 0
Merge step 2: Combine connected gmaps. Consider two elements of Gmnaps, which share base structure.i.e..whose roots in the base structure are identical. Since we are assuming distinct elements, either (a) their correspondences are structurally inconsistent or (b) there is some structure in the base which connects them that does not appear in the target (if it did, match hypotheses would have been created which would bring the two elements under a common match hypothesis root; hence they would not be distinct). Combining such elements, when consistent, leads to potential support for candidate inferences. We call the partial gmaps resulting from this merge Gmaps_ (Fig.6(b)).
Merge step 3: Combine independent collections. Consider two elements of Gmaps, which have no overlap between their relational correspondences .
Clearly, any such pair could be merged without inconsistency, if they sanction consistent sets of emaps. This final step generates all consistent combinations
of gmaps from Gmaps, by successive unions, keeping only those combinations that are maximal (Fig .6(c)).
Example (Simple analogy between heat and water) . Figure 6 shows how the gmaps are formed from the collection of match hypotheses for the simple water flow/heat flow example . After merge step 1, only isolated collections stemming from common roots exist. Merge step 2 combines the PRESSURE toTEMPERA- TURE mapping with the FLOW mapping, since they have common base structure (i.e., the base structure root is the CAUSE predication). Finally, merge step 3 combines the isolated water and coffee attributes (see Fig . 7). Notice that the FLOW mapping is structurally consistent with the DIAMETER to TEMPERATURE mapping. However, because merge step 2 placed the FLOW mapping into the same gmap as the PRESSURE toTEMPERATURE mapping, merge step 3was unable to combine the FLOW mapping with the DIAMETER to TEMPERATURE gmap.
3.2.3. Compute candidate inferences (Step 3)
Each gmap represents a set of correspondences that can serve as an interpreta- tion of the match. For new knowledge to be generated about the target, there must be information from the base which can be carried over into the target . Not just any information can be carried over-it must be consistent with the substitutions imposed by the gmap, and it must be structurally grounded in the gmap . By structural grounding, we mean that its subexpressions must at some

22
B. FALKENHAINER ET AL . lumbar of !latch Hypotheses : 14
Rule File : literal-similarity .rules Match Hypotheses :
(0.6500 0.0000) (>PRESSURE TEMP)
(0.7120 0.0000)
(0.7120 0.0000) (PRESS-VIAL TEMP-ICE-CUBE)
(0.9316 0.0000) (BEAKER-6 COFFEE-1) (0.6320 0.0000) (PIPE-8 BAR-3)
000 000
Global Mappings :
Gasp *I : (>PRESSURE TEMPERATURE) (PRESSURE-BEAKER TEMP-COFFEE) (PRESSURE-VIAL TEMP-ICE-CUBE) (WFLOW HFLOW)
Esaps: (beaker coffee) (vial ice-cube) (eater heat) (pipe bar) Weight: 6.99
Candidate Inferences : (CAUSE >TEMPERATURE HFLOW)
Gasp 82 : (DIAMETER >TEMPERATURE) (DIAMETER-1 TEMP-COFFEE) (DIAMETER-2 TEMP-ICE-CUBE)
taaps: (beaker coffee) (vial ice-cube) Weight: 3.94
Candidate inferences :
Cup 83: (LIQUID-3 LIQUID-6) Laps : (water coffee) Wight : 2.44
Candidate Inferences :
(FLIT-TOP-4 FLAT-TOP-9)
(PRESS-BEAKER TEMP-COFFEE)
Fig. 7. Complete SME interpretation of water flow/heat flow analogy .
point intersect the base information belonging to the gmap . Such structures form the candidate inferences of a gmap .
To compute the candidate inferences for a gmap GM, SME begins by examining each root BR in the base dgroup to see if it is an ancestor of any match hypothesis roots in the gmap . If it is, then any elements in Descen-
dants(BR)which are not in Baseltems(GM) are included in the set of candidate inferences.
The candidate inferences often include entities. Whenever possible, SME replaces all occurrences of base entities with their corresponding target entities .
Sometimes, however, there will be base entities that have no corresponding
target entity ; i.e., the base entity is not part of any match hypothesis for that gmap. What SME does depends on the type of entity. If the base entity is a
constant, such as zero, it can be brought directly into the target unchanged (a flag is provided to turn on this behavior). Otherwise.SME introduces a new, hypothetical entity into the target which is represented as a skolem function of the original base entity . Such entities are represented as ( :skolem base-entity).
Recall that structure-mapping does not guarantee that any candidate infer-

THE STRUCTURE-MAPPING ENGINE
23
ence is valid. Each candidate inference is only a surmise, which must be tested by other means. By theoretical assumption. general testing for validity and relevance is the province of other modules which use SMEs output. However. SME does provide a weak consistency check based on purely structural conside-
rations . In p articular . it discards a candidate inference when (a) the predicate is
noncommutative and (b) its argum..nts are simply a permuted version of the arguments to another expression involving that predicate in the target domain . For example . if (GREATER (MASS sun) (MASS planet)) existed in the target. (GREATER (MASS planet) (MASS sun)) would be discarded as a candidate inference.
Example (Simple analogy between heat and water) . In Fig. 7, gmap # 1 has the top-level CAUSE predicate as its sole candidate inference . In other words, this gmap suggests that the cause of the flow in the heat dgroup is the difference in temperatures .
Suppose the FLOW predicate was missing in the target dgroup . Then the candidate inferences for a gmap corresponding to the pressure inequality would include expressions involving both CAUSE and FLOW, as well as conjectured target entities corresponding to water (heat) and pipe (bar) . The two skolem- ized entities would be required because the FLOW match provides the match from water and pipe to heat and bar, respectively . Note also that GREATER- THAN(DIAMETER(coffee), DIAMETER(ice-cube)) is not a valid candidate inference for the first gmap because it does not intersect the existing gmap structure .
3.2.4. Compute structural evaluation scores (Step 4)
Typically a particular base and target pair will give rise to several gmaps, each representing a different interpretation. Selecting the best interpretation of an analogy, as mentioned previously, can involve nonstructural criteria . How- ever, as the psychological results indicated, evaluation includes an important structural component.SME provides a programmable mechanism for computing a structural evaluation score (SES) for each gmap . This score can be used to rank-order the gmaps or as a factor in some external evaluation procedure .
The structural evaluation score is computed in two phases, each using match evidence rules to assign and manage numerical scores . The first phase assigns weights to individual match hypotheses, and the second phase computes a score for each gmap by combining the evidence for the match hypotheses comprising its correspondences. After a brief introduction to the evidence processing mechanism, we describe each phase in turn .
The management of numerical evidence is performed by a belief maintenance system (BMs) [161. The BMS is much like a standard TMs, using Horn clauses as
justifications. However, the justifications are annotated with evidential One such module is described in [17.20).

24
B. FALKENHAINER ET AL .
w eights . so that degrees of belief may be propagated . A modified version of Dempster-Shafer formalism is used for expressing and combining evidence .
Belief in a proposition is expressed by the pair (s(A), s(-A)), where s(A) represents the current amount of support for A and s(-iA) is the current support against A. A simplified form of Dempsters rule of combination [16, 39 .57.64] allows combining evidence from multiple justifications. For example, given that Belief(A) = (0.4,0) and Belief(B) _ (0.6,0), together with (IMPLIES A C)(0e.0) and (IMPLIES B C),,., Dempsters rule provides a belief in C equal to (0 .728, 0.0). In addition to providing evidence combination, these justifications provide useful explanations about the structural evaluation (see [16]).
We note two points about the role of numerical evidence in SME :
(1) While we have found Dempster-Shafer, useful, our algorithms are independent of its details, and should work with any reasonable formalism for combining evidence.
(2) We use numerical evidence to provide a simple way to combine local information concerning match quality . These weights have nothing to do with any probabilistic or evidential information about the base or target per se.
Assigning local evidence
Each match hypothesis and gmap has an associated BMS node to record evidential information. The match evidence rules can add evidence directly to a match hypothesis based on its local properties or indirectly by installing relationships between them. Syntactically, these rules are similar to the match constructor rules. For example,
(assert! same-functor) (rule ((:intem (MH ?b ?t)
:test (and (expression? ?b) (expression? ?t) (eq (expression-functor ?b)
(expression-functor ?t)))))
(assert! (implies same-functor (MH ?b,?t) (0.5.0.0))))
states that if the base item and target item of a match hypothesis are expressions with the same functors, then supply 0.5 evidence in favor of the match hypothesis. (The assertion of same-functor provides a global record for explanatory purposes that this factor was considered in the structural evalua- tion.) The complete set of evidence rules used in this paper are provided in Appendix A.
The ability to install relationships between match hypotheses allows a simple, local implementation of the systematicity constraint. Recall that the systematicity constraint calls for preferring expressions involving higher-order relationships belonging to a systematic structure over isolated relationships . We

THE STRUCTURE-MAPPING ENGINE 25
implement this preference by passing evidence from a match involving a relationship to the matches involving its arguments . The following rule accom- plishes this. propagating 80% of a match hypothesis belief to its offspring :
(rule((:intern(MH ?bl ?t1))
(:intern (MH ?b2 ?t2) :test (children-of? ?b2 ?t2 ?b1 V)))
(assert!(implies(MH ?bt?t1)(MH ?b2?t2)(0.8.0.0)
The more matched structure that exists above a given match hypothesis, the more that hypothesis will be believed. The effect cascades, so that entity mappings involved in a large systematic structure receive much higher scores than those which are not. Thus this trickle down effect provides a local encoding of the systematicity principle.
Computing the structural evaluation score
The structural evaluation score for a gmap is simply the sum of the evidence of its match hypotheses. This simplistic summation rule has sufficed for most of the examples encountered so far . There are a number of other factors that are potentially relevant as well, which we discuss in Section 7 .3.1. In order to provide maximum flexibility, evidence rules are used to compute the evidence of gmaps as well.
Originally we combined evidence for gmaps according to Dempsters rule, so that the sum of beliefs for all the gmaps equaled 1 [21].We discovered two problems with this scheme . First. Demsters rule is susceptible to roundoff. which caused stability problems when a large number of match hypotheses supported a gmap. Second, normalizing gmap evidence prevents us from comparing matches using different base domains (as needed for access experi- ments, see Section 4.4), since the score would be a function of the other gmaps for a particular base and target pair.
Example (Simple analogy between heat and water) . Returning to Fig. 7. note that the best interpretation (i.e. the one which has the highest structural
evaluation score) is the one we would intuitively expect. In this interpretation. beaker maps to coffee, vial maps to ice-cube, water maps to heat, pipe maps to bar. and PRESSURE maps to TEMPERATURE . Furthermore, we have the candidate inference that the temperature difference is what causes the flow of heat .
3.3. Complexity analysis
Here we analyze the complexity of the SME algorithm. Because it depends critically on both the input descriptions and the match rules, strict bounds are
hard to determine. However. we give both best- and worst-case analyses for each step, and provide estimates of typical performance based on our ex-

26 B. FALKENHAINER ET AL.
1. Run MHC rules to construct match hypotheses .
2. Calculate the Conflicting set for each match hypothesis.
3. Calculate the EMaps and NoGood sets for each match hypothesis by upward propagation from entity mappings.
4. Merge match hypotheses into gmaps. (a) Interconnected and consistent.
(b) Consistent members of same base structure .
(c) Any further consistent combinations .
5. Calculate the candidate inferences for each gmap . 6. Score the matches
(a) Local match scores.
(b) Global structural evaluation scores.
Fig. 8. Summary of SME algorithm .
perience . The decomposition used in the analysis is shown in Fig . 8. We use the following notation in the analysis:
Wb = number of entities in the base dgroup,
i6t= number of entities in the target dgroup,
S;b -number of expressions in the base dgroup. ,9,a number of expressions in the target dgroup,
Al a number of match hypotheses,
19 -number of gmaps,
Nb=Wb+ 4b, Nts9,+At, Nw (Nb+N,).
3.3.1. Analysis of Step 1:Local match construction
SME does not restrict either the number of match rules or their complexity . There is nothing to prevent one from writing a rule that examines extensive
information from external sources (e.g., a knowledge base, plans, goals, etc.). However, the rule sets which implement the comparisons of structure-mapping theory consist of only a few simple rules each. This reduction of computational
complexity is one of the advantages of the structure-mapping account, since it restricts the tests performed in rules to local properties of the representation . Consequently, we assume rule execution takes unit time, and focus on the total number of rules executed. The :filter rules are run for each pair of base and targetpredicates.Consequently,theywillalwaysrequireO(N,,-.N,)Each antem rule is run once on every match hypothesis. In the worst case, X = N h -N,, or roughly N 2.But in practice, the actual number of match hypotheses is substantially less, usually on the order of cN, where c is less than 5 and N is the average of Nband N,.Thus, in practice, Intern rules have a run time of approximately O(N).

THE STRUCTURE-MAPPING ENGINE 27
3.3.2. Analysis of Step 2: Calculating Conflicting
Recall that SME assigns a Conflicting set to each match hypothesis, MH(b,, t.). which represents the alternate mappings for b, and t,.The conflicting sets are
calculated -by examining each base and target item to gather the match hypotheses which mention them . Let g be the average number of alternative matches each item in the base and target appears in.SME loops through the match hypotheses twice: once to form the bitwise union of these match hypotheses and once to update each hypothesis Conflicting set. Thus, the entire number of bit vector operations is
(3`b-2`g)+(9t,-216)+(x,-28)+ (Wt-2C) .
The worst case is when a match hypothesis is created between every base and target item . If we also assume N b = N,, then Ce = N, in that case . The
number of operations becomes 4N;or approximately O(N).Conversely, the best-case performance occurs when C is 1, producing O(max(N b, N,)) oper- ations. In our experiments so far, we find that Ceis typically quite small, and so far has always been less than 10. Consequently, the typical performance lies betweenO(N) andO(N2).
3.3.3. Analysis of Step 3: Emaps and NoGood calculation
Recall that once the Conflicting sets are calculated, the Emaps and NoGood sets are propagated upwards from the entity mappings through the match hypotheses. By caching which MH(b;,t,)scorrespond to emapsand using a queue, we only operate on each node once . Hence the worst- and best-case performance of this operation is O(A), which in the worst case is O(N2).
3.3.4. Analysis of Step 4: Gmap construction
Global matches are formed in three steps. The first step collects all of the consistent connected components of match hypotheses by starting at the match hypothesis roots, walking downwards to find consistent structures . Each graph walk takes at most O(N;), where N ;is the number of nodes Reachable from the current match hypothesis root. If there are N R roots, then the first merge step (Step 4(a)) takes O(NR N ;). Assuming that most of the match hypotheses will appear in only one or two subgraphs (some roots may share substructure) . we can approximate this by saying that the first merge step is 0(,M). Call the number of partial gmaps formed at this stage &P,.
Perhaps surprisingly, the complexity of the previous steps has been uniform- ly low. Sophisticated matching computations usually have much worse perfor- mance, and SME cannot completely escape this . In particular. the worst case for Steps 4(b) and 4(c) is O(N!) (although worst case for one implies best case for the other).

28 B. FALKENHAINER ET AL.
Step 4(b) combines partial gmaps from Step 4(a) that intersect the same base structure. This requires looping through each base description root to find which partial gmaps intersect it, and then generating every consistent, maximal combination of them. In the worst case, every gmap could intersect the same base structure. This would mean generating all possible consistent, maximal sets of gmaps, which is equivalent to Step 4(c).so we defer this part of the analysis until then. In the other extreme, none of the gmaps.share a common base structure, and so Step 4(b) requires 0( 19 :1) operations, although this is not the best-case performance (see below) . Typically, the second merge step is very quick and displays near best-case performance .
Step 4(c) completes gmap construction by generating all consistent combina- tions of the partial gmaps, discarding those which are not maximal . The complexity of this final merge step is directly related to the degree of structure in the base and target domains and how many different predicates are in use . Worst-case performance occurs when the description language is flat (i.e..no higher-order structure) and the same predicate occurs many times in both the base and the target . Consider a language with a single, unary predicate, and base and target dgroups each consisting of N distinct expressions. Inthis case
every base expression can match with every target expression, and each such match will suggest matching in turn the entities that serve as their arguments .
This reduces to the problem of finding all isomorphic mappings between two equal size sets, which is O(N!) .
Now let us consider the best case . If the base and target dgroups give rise to a match hypothesis graph that has but one root, and that root is consistent, then there is only one gmap! The second and third merge steps in this case are now independent of N, i.e., constant-time.
Of course, the typical case is somewhere between these two extremes . Typically the vocabulary of predicates is large. and the relationships between entities diverse. Structure provides a strong restriction on the number of possible interpretations for an analogy.By the time SME gets to Step 4, many of the match hypotheses have been filtered out as being structurally impossible . Steps 4(a) and 4(b) have already merged many partial gmaps, reducing the number of elements which may be combined . The identicality constraint of structure-mapping (encoded in the match rules) also reduces typical-case complexity, since match hypotheses are only created between relations when functors are identical. Thus, SME will perform badly on large descriptions with no structure and extensive predicate repetition, but will perform well on large descriptions with deep networks of diverse higher-order relationships . Semanti- cally, the former case roughly corresponds to a jumble of unconnected expressions, and the latter case to a complex argument or theory . The better
organized and justified the knowledge, the better SME will perform.
While the potential complexity of Step 4(b) is O(N!), our experience is that this step is very quick and displays near best-case performance in practice . We

THE STRUCTURE-MAPPING ENGINE 29
suspect the worst-case behavior is very unlikely to occur, since it requires that all members of Gmaps, intersect the same base structure and so must be merged in all possible ways. However. partial gmaps intersecting the same base structure are almost always consistent with one another, meaning that Step 2 would usually merge Gmaps, into one gmap in O(IgP,) time. On the other hand, we have on occasion experienced worst-case performance for Step 4(c).
3.3.5. Analysis of Step 5: Finding candidate inferences
The candidate inferences are gathered by looping through the base description roots for each gmap, collecting missing base expressions whenever their structure intersects a match hypothesis in the gmap . Each expression is tested to ensure that (1) it is not already matched with part of the target description, and (2) that it does not represent a syntactic contradiction of an existing target expression. The size of the typical candidate inference is inversely related to the percentage of base structure roots: more roots implies less structure to infer, and vice versa . Thus in the worst case we have O(19- ;Fb 3;,), or roughly O(N ;). However, this is an extreme worst case. First, the 3;,term implies that we check every target expression on each iteration. The algorithm actually only checks the pertinent target expressions (i.e., those with the same predicate), giving a tighter bound of O(N ).In the best case, there will only be one gmap and no candidate inferences, producing constant-time behavior .
3.3.6. Analysis of Step 6:SES computation
The complexity of the BMS is difficult to ascertain. Fortunately it is irrelevant to our analysis since the BMS can be eliminated if detailed justification of evidential results are not required. For example, the first version of SME (211 used specialized evidence rules which had most of the flexibility of the BMS-based rules yet ran in O(.,R) time.
Although the flexibility of the BMS can be valuable, in fact the majority of SMES processing time takes place within it-typically 70 to 80% . So far this has not been a serious performance limitation, since on the examples in this paper (and most of the examples we have examined) .SME takes only a few seconds on a Symbolics machine .
4. Examples
The structure-mapping engine has been applied to a variety of domains and tasks. It is being used in psychological studies, comparing human responses with those of SME for both short stones and metaphors.SME is serving as a module in a machine learning program called PHINEAS, which uses analogy to discover and refine qualitative models of physical processes such as water flow and heat flow.SME is also used in a concept-learning program called SEOL .

30 B. FALKENHAINER ET AL .
which induces structural descriptions from examples (66, 68] . Here we discuss a few representative examples to demonstrate SMEs flexibility and generality .
4.1. Methodological constraints
Flexibility is a two-edged sword . The danger in using a program like SME is that one could imagine tailoring the match construction and evidence rules for each new example . Little would be learned by using the program in this way-we would have at best a series of wind-up toys, a collection of ad hoc programs which shed little theoretical light . Here we describe * our techniques for reducing tailorability.
First, all the cognitive simulation experiments were run with a fixed collec- tion of rule sets, listed in Appendix A . Each rule set represents a particular type of comparison sanctioned by the structure-mapping theory (i.e.,analogy,
literal similarity, and mere appearance) . The mere-appearance rules (MA) match only low-order items : attributes and first-order relations . The analogy rules (AN) match systems of relations and higher-order relations, while the
literal similarity rules (LS) match both low-order and higher-order structure . The first two examples in this section use theAN rules, and the third uses both AN and MA rules, as indicated.
While the choice of match construction rules is dictated by structure- mapping, the particular values of evidence weights are not . Although we have not performed a sensitivity analysis, in our preliminary explorations it appears that the gmap rankings are not overly sensitive to the particular values of evidence weights. (Recall that which gmaps are constructed is independent of the weights, and is determined only by the construction rules and structural consistency.)
Second, we have accumulated a standard description vocabulary which is used in all experiments. This is particularly important when encoding natural language stories, where the translation into a formal representation is under- constrained. By accumulating representation choices across stories, we attempt to free ourselves from biasing the description for particular examples .
Third.we have tested SME with descriptions generated automatically by other Al programs. A representation developed to perform useful inferences
has fewer arbitrary choices than a representation developed specifically for learning research. So far, we have used descriptions generated by two different
qualitative simulation programs with encouraging results. For example.W E actually performs better (in the sense of producing fewer spurious interpreta- tions) on a water flow/heat flow comparison using more complex descriptions generated by GIZMO [241 than it does on many hand-generated descriptions. We are working on interfacing SME to other inference systems, as described in Section 7.3.1.

THE STRUCTURE-MAPPING ENGINE 31
4.2. Solar system/ Rutherford atom analogy
The Rutherford model of the hydrogen atom was a classic use of analogy in science. The hydrogen atom was explained in terms. of the better understood
behavior of the solar system. We illustrate SMEs operation on this example with a simplified representation, shown in Fig . 9.
SME constructed three possible interpretations. The highest-ranked mapping (SES = 6.03) pairs the nucleus with the sun and the planet with the electron .
This mapping is based on the mass inequality in the solar system playing the same role as the mass inequality in the atom . It sanctions the inference that the differences in masses, together with the mutual attraction of the nucleus and the electron, causes the electron to revolve around the nucleus. This is the standard interpretation of this analogy .
The other major gmap (SES = 4.04) has the same entity correspondences . but maps the temperature difference between the sun and the planets onto the mass difference between the nucleus and the electron. The SES for this gmap is low for two reasons. First, temperature and mass are different functions, and hence they receive less local evidence . The second, and more important reason is that there is no mappable systematic structure associated with temperature in the base dgroup. Thus other relations, such as the match for ATTRACTS, do not enter into this gmap. We could in theory know a lot more about the thermal properties of the solar system than its dynamics, yet unless there is some relational group in the target description there will not be a set of mappable systematic relations. (If we instead were explaining a home heating system in terms of the solar system the situation would be the reverse .)
The third gmap is a spurious collection of match hypotheses which imply that the mass of the sun should correspond to the mass of the electron, and the
mass of the planet should correspond to the mass of the nucleus . There is even less structural support for this interpretation (SES = 1.87).
This example demonstrates an important aspect of the structure-mapping
SOUR SYSTEM
CA1JSE
CAUSE AND REVOLVE(Manet.sun)
GRAVITY ATTRACTS(sun.plmat) GREATER
MASS(sun) MASS(Oanst)
GR TER TEMPERATURE(vin) TEMPERATURErn4anal)
RUTHERFORD ATOM
CAUSE
1- -1
OPPOSITE~-SIGN ATTRACTS(M+c/aus.a/actron)
CHARGE( nucleus) CHARGE(electron)
Fig. 9. Solar system/ Rutherford atom analogy.
REVOLVE (r.ciron. nucleus)
MASS(nudaus) MASS(elsctron)
/1

32 B. FALKENHAINER ET AL.
account of analogy . The interpretation preferred on structural grounds is also the one with the most inferential import . This is not an accident ; the sys- tematicity principle captures the structural features of well-supported argu- ments. Using the structure-mapping analogy rules (AN), SME prefers interpreta- tions based on a deep theory (i.e., a subset ofa dgroup containing a system of higher-order relations) to those based on shallow associations (i.e., a subset of a dgroup containing an assortment of miscellaneous facts) .
4.3. Discovering heat flow
The PHINEAS program [17, 18, 20] learns by observation. When presented with a new behavior, it attempts to explain it in terms of its theories of the world. These theories are expressed as qualitative models of physical processes using Forbus qualitative process theory [22, 23].When it is given a behavior that it cannot explain, an analogical learning module is invoked which attempts to generate a new or revised model that can account for the new observation . This module uses SME in two ways.5FirstSME is used to form a match between a previous experience which has been explained and the current behavior. These correspondences provide the foundation for constructing a model that can explain the new observation based on the model for the previous behavior .
For example, suppose that the program was presented with measurements of the heat flow situation depicted in Fig. 10 and described in Fig. 11. If the domain model does not include a theory of heat flow, PHINEAS will be unable to interpret the new observation 6 Using SME, PHINEAS constructs an analogy with the previously encountered water flow experience also shown in Figs. 10 and 11 . This match establishes that certain properties from the two situations behave in the same way. As shown in Fig. 11, the roles of the beaker and the vial in the water flow history are found to correspond to the roles of the horse shoe and water in the heat flow history, respectively . PHINEAS stores the
Fig. 10. Two examples of water flow and heat flow .
` In this example PHINEAS is using the structure-mapping analogy rules . In normal o peration . it
uses a more knowledge-intensive rule set that relaxes the identicality constraint and includes
goal-sensitive match evaluation criteria (201 . . See PIIINEAS uses the ATMI theory of measurement interpretation to explain observations
1251 for details .

THE STRUCTURE-MAPPING ENGINE
33
Water-Flow History
(Situation SO)
(Decreasing (Pressure (At beaker SO))) (Increasing (Pressure (At vial SO))) (Decreasing (Amount-of (At beaker SO))) (Increasing (Amount-of (At vial SO))) (Greater (Pressure (At beaker S0))
(Pressure (At vial SO)))
(Situation Si)
(Meets SO S1)
(Constant (Pressure (At beaker S1))) (Constant (Pressure (At vial S1))) (Constant (Amount-of (At beaker S1))) (Constant (Amount-of (At vial SI))) (Equal-To (Pressure (At beaker S1))
(Pressure (At vial S1)))
(Function-Of (Pressure x) (Amount-of x))
Heat-Flow History
(Situation SO)
(Decreasing (Temp (At horse-shoe SO))) (Increasing (Temp (At water SO))) (Greater (Temp (At horse-shoe SO))
(Temp (At water SO)))
(Situation St) (Meets 50 St)
(Constant (Temp (At horse-shoe S1))) (Constant (Temp (At eater S1))) (Equal-To (Temp (At horse-shoe SI))
(Temp (At water S1)))
(Function-Of (Temp x) (Heat x))
Behavioral Correspondences
Pressure Temperature Amount-of Host
so so
31 51
beaker horse-shoe
vial water
Fig. 11. Analogical match between water flow history and heat flow history.
correspondences that provide a mapping between entities or between their quantities (e.g.,Pressure and Temperature) for later reference.
When it is satisfied that the chosen water flow history is sufficiently analog- ous to the current situation, PHINEAS begins a deeper analysis of the analogy.
It fetches the domain used to generate its prior understanding of the base (water flow) experience . Its description of water flow, shown in Fig . 12, is a straightforward qualitative model similar to that used in other projects [24 .27J. This model states that if we have an aligned fluid path between the beaker and the vial (i.e., the path either has no valves or if it does, they are all open) . and the pressure in the beaker is greater than the pressure in the vial, then a liquid flow process will be active. This process has a flow rate which is proportional to the difference between the two pressures . The flow rate has a positive influence on the amount of water in the vial and a negative influence on the amount of
water in the beaker.
Using SME a second time, this theory is matched to the current heat flow
situation using the correspondences established with the behavioral analogy . The output is shown in Fig. 13. The entity and function correspondences

34
B. FALKENHAINER ET AL .
Path
PRESSLRV AJIOC`T-OP
PRESSLRE AMOUNT-OP
T Source
,
Fig. 12. Qualitative process theory model of liquid flow .
provided by the behavioral analogy provide significant constraint for carrying over the explanation.SMEs rule-based architecture is critical to this operation : PHINEAS imposes these constraints by using a special set of match constructor rules that only allow hypotheses consistent with the specific entity and function correspondences previously established . Entities and functions left without a match after the accessing stage are still allowed to match other unmatched entities and functions. For example, the rule
(MHC-rule ( :filter ?b ?t
:test (sanctioned-pairing? (expression-functor ?b)
(install-MH ?b ?t))
(expression-functor ?t)))
forces a match between those quantities which were found to be analogous in the behavioral analogy (e.g.,PRESSURE and.TEMPERATURE) and prevents
6aap $I : ( (AMOUNT-OF-35 HEAT-WATER) (AMOUNT-OF-33 HEAT-HSHOE) (PRESSURE-DEARER TEMP-HSHOI) (PRESSURE-VIAL TEMP-WATER) }
Rasps: ( (Maker hers.-shoo) (vial vat.r) } Weight : 2 .675
Candidat . Inferences : (IMPLIES
(AHD (ALIGNED (:skol.a pip.))
(GREATER-TEAS (A TEMP-RSHOE) (A TEMP-WATER)))
(AND (Q (FLOW-RATE pi) (- TEMMHSHOE TEMP-WATER)) (GREATER-THAN (A (FLOW-RATE pi)) s.ro) (IHEAT-WATER (A (FLOW-RATE pi)))
(I- HEAT-HSHOR (A (FLOW-RATE pi)))))
Fig. 13. At. analogically inferred model of heat flow.
Destination

THE STRUCTURE-MAPPING ENGINE 35
any alternate matches for these quantities (e.g., AMOUNT-OF and TEM- PERATURE) .
This example demonstrates several points . First, the second analogy, which imports the theoretical explanation of the new phenomena, is composed almost entirely of candidate inferences, since the system had no prior model of heat flow. It is largely a carryover analogy [31]. Hence, the model was constructed by analogy rather than augmented by analogy. This shows the power of SMEs candidate inference mechanism . Second, -the example illustrates how SMEs rule-based architecture can support tasks in which the entity correspondences are given prior to the match, rather than derived as.a result of the match. Finally, it shows the utility of introducing skolemized entities into the candi- date inferences. The results produced by SME (Fig. 13) contain the entity (:skolem pipe). This indicates that, at the moment, the heat path is a conjec- tured entity. At this time, the system inspects its knowledge of paths to infer that immersion or physical contact is a likely heat path. However, we note that much knowledge gathering and refinement may still take place while leaving the heat path as a conjectured entity. For example, in the history of science ether was postualted to provide a medium for the flow of light waves because other kinds of waves require a medium.
4.4. Modeling human analogical processing
SME is being used in several cognitive simulation studies. Our goal is to compare SMEs responses with those of humans for a variety of tasks and
problems. For example, two psychological studies [35, 59] have explored the variables that determine the accessibility of a similarity match and the inferen- tial soundness of a match. Structure-mapping predicts that soundness is de- termined by the degree of systematic relational overlap [30]. In contrast, Gentner [31, 321 has suggested that the accessibility of potential matches in long-term memory is heavily influenced by surface similarity . Psychological studies have supported both hypotheses [35, 59, 62] . In order to verify the computational assumptions we ran SME on the same examples that had been given to human subjects. Here we briefly summarize the simulation methodolo-
gy and the results; for details see [67]. . Pairs of short stories The hypotheses were tested psychologically as follows
were constructed which were similar in different ways : in particular, some pairs embodied mere appearance and some analogy . Subjects read a large set of stories, which were the base members of each pair. Then, in a second session, subjects saw the similar stories and tried to retrieve the original stories (the access measure) . After that, the subjects were then asked to judge the inferential soundness of each of the story pairs. For the cognitive simulation
Other kinds of matches. including literal similarity, were also used. Here we discuss only analogy and mere appearance.

36
B. FALKENHAINER ET AL.
study. five triads of stories-a base, a mere-appearance match, and an analogy match were encoded (15 in all). The pairs of stories were presented to SMtE,
using different rule sets corresponding to analogy (the A N rules) and mere appearance (the M A rules). The results from the A N rules were used to estimate soundness, while the results from the M A rules were used to estimate acces- sibility. One of these story groups will be discussed in detail . showing how SMME was used to simulate a test subject.
In the story set shown in Fig. 14, the original story concerned a hawk named Karla who survives an attack by a hunter. Two target stories were used as potential analogies for the Karla narration . One was designed to be truly analogous (TA5) and described a small country named Zerdia that survives an attack by another country. The other story (MA5) was designed to be only superficially similar and described an eagle named Zerdia who is killed by a sportsman. The representation of the Karla story given to SME was:
(CAUSE (EQUALS (HAPPINESS HUNTER) HIGH)
(PROMISE HUNTER KARLA (NOT (ATTACK HUNTER KARLA))))
(CAUSE (OBTAIN HUNTER FEATHERS) (EQUALS (HAPPINESS HUNTER) HIGH)) (CAUSE (OFFER KARLA FEATHERS HUNTER) (OBTAIN HUNTER FEATHERS)) (CAUSE (REALIZE KARLA (DESIRE HUNTER FEATHERS))
(OFFER KARLA FEATHERS HUNTER))
Base Story
Karla, an old hawk, lived at the top of a tall oak tree. One afternoon, she saw a hunter on the ground with a bow and some crude arrows that had no feathers.The hunter took aim and shot at the hawk but missed. Karla knew that hunter wanted her feathers so she glided down to the hunter and offered to give him a few. The hunter was so grateful that he pledged never to shoot at a
hawk again. He went off and shot deer instead.
Target Story: Analogy . Once there was a small country called Zerdia that learned to make the worlds smartest computer
One day Zerdia was attacked by its warlike neighbor, Gagrach. But the missiles were badly aimed and the attack failed. The Zerdian government realized that Gagrach wanted Zerdian computers so it offered to sell some of its computers to the country. The government of Gagrach was very pleased. It promised never to attack Zerdia again.
Target Story: More Appearance
Once there was an eagle named Zerdia who donated a few of her taifeathers to a sportsman so
he would promise never to attack eagles .
One day Zerdia was nesting high on a rocky cliff when she saw the sportsman ox,*g with a
crossbow. Zerdia flew down to meet the man, but he attacked and felled her with a single bolt.As she fluttered to the ground Zerdia realized that the bolt had her own tailfeathers on it.
Fig. 14. Story set 5.

THE STRUCTURE-MAPPING ENGINE 37
(FOLLOW (EQUALS (SUCCESS (ATTACK HUNTER KARLA)) FAILED) (REALIZE KARLA (DESIRE HUNTER FEATHERS)))
(CAUSE (NOT (USED-FOR FEATHERS CROSS-BOW))
(EQUALS (SUCCESS (ATTACK HUNTER KARLA)) FAILED))
(FOLLOW (SEE KARLA HUNTER) ATTACK HUNTER KARLA)) (WEAPON CROSS-BOW)
(KARLAS-ASSET FEATHERS)
(WARLIKE HUNTER)
(PERSON HUNTER) (BIRD KARLA)
The results from human subjects showed that (1) in the soundness evaluation task, as predicted by Gentners systematicity principle, people judged analogies as more sound than mere-appearance matches ; and (2) in the memory access task, people were more likely to retrieve surface similarity matches than analogical matches.
To test SME as a cognitive simulation of how people determine the soundness of an analogy, SME was run using its analogy (AN) match rules on each base-target pair of stories-that is, base/ mere-appearance story and base/ analogical story. Figure 15 shows the output of SME for the AN task. For
Analogical Match from Karla to Zerdia the country (TAS) . Sale file : aaaleh.rales #=hot .1 )Easel typetlassas: 14 Hshsr of Guava : I
*map It:
(crust-PROfhsE CAUSE-PtORISS) (SULCUs-aTTaes SUCCIM$-ATTICS) (IAPPY-IVTTU SAPPY-GAGUCI) (IAPPZSUSS-IIITLR UPPISESS-GISUCI) ()(ULIit-0911111S SULIU-DEIRt) (CAUSE-TARS CAUSC-IVY) (ATTICS-MTU ATTIC&-GIOtICI) (DCSlRU-FUTUts DESIRE-SUPt3COtOVTU) (F12LZD-4TTACZ FAILZD-ATTACK) (Ta&t-UaTUURS IVY-SUPURCOJOUTU) (CAU$t-FASLED-ATTIC& CAVit-PSILLD-ATTICS)
(C8VSI-OFFER CAUSZ-0F7IR) (FOLLOW-DULSU FOLLOW-LULIZZ) (US-PUTIUS Vat-SUPURCOIPVTU) (CAUSEISPPY CAUSE -SAPPY) (SOT-ATTICS SOT-ATTIC*) (PROMISE-lUiTZR P105251)
(NOT-US-PIATIUS SOT-USE-$VPUCOUPUTU) (OPTU-PEITISRS OrTUR-SUPERCORPUTfl)
Swaps: (520123 110811) (FUTEEUIO SUPUCO)VWM14) (CROSS-10V21 MI5SIL1311) (SVITUIS GAGLC113) (1AUL11M 251DIau) (?81M22 PA2L1215)
Jeitie: 22.342716
Analogical Match from liana to Zerdia the eagle (M45).
Sale file : aaalap .ralss timber ei Neigh 11Peeleses : 47 lager at Guava : i
Gasp Ii:
(PROMISE-1141M P525231) (DESItt-PUTIERM OESIRE-FEITIW) (TARR-FILTUU TARE-FUTIUS) (CAUSE-OFlSR CAUSE-OFPU) (OPPU-PUTURS OPPRR-PUTURS) (M-IUTIUS 5AM-PUTURS) (RULIZZ-DSUtt RULIZZ-0t32RE) (ATTACK-WITH ATTICS-SPORTSRIl) (SOT-*TTACR SOT-aTTICR) (SUCCESS-ATTICS SUCCESS-&TTSCZ) (POLL05-599-ATTACK FOLLOW-alt) (StPEiRLI SEE-ZZUII) (FIILSD-ATTAGI SUCCUSFUL-ATTIC&) (CAUSE-T85 CAUSE-TAtt)
Leaps : (F8ILED23 TRUU1) (IIILAIS ZUDIA7) (IUITtl1! $POIT$1AI4)
(FUTIU320 MUMS) (Cl0$$-10V21 CROSS-WVIO) Weight : 14 .414930
Fig. 15. SMEs analysis of story set 5 . using the TA rules.

38 B. FALKENHAINER ET AL .
example. Zerdia the country (the analogy) was found to be a better
analogical match (SES = 22.4) to the original Karla story than Zerdia the
eagle (SES = 16.8). Overall, SME as an analogical mapping engine agrees quite well with the soundness rating of human subjects .
We also used SME to test the claim that the human access patterns result from access depending on surface similarity matches (objects and object- attribute overlap).SME was run on each of the pairs using its mere-appearance (MA) match rules. This measured their degree of superficial overlap. Again. over the five stories SMEs rankings match those of human subjects. For example, the output of SME for the MA task is given in Fig. 16, which shows that the eagle story (SES = 7.7) has a higher MA rating than the country story (SES = 6.4).
It should be noted that, unlike the soundness rating task, the access mimicking task is not a true simulation. To do this would require finding and selecting the prior story from a large set of potential matches. Rather, SME is acting as a bookkeeper to count the variable (here, degree of surface overlap) being claimed as causally related to the variable being measured (accessibility of matches). The results demonstrate that surface similarity, as strictly defined and used in SMEs match rules, matches well with peoples retrieval patterns in an access task. In contrast, in the soundness rating simulation, SMEs analogy processes constitute a psychologically viable model.
This study illustrates the viability ofSME as a cognitive simulation of human processing of analogy . We make two additional observations . First, the results demonstrate the considerable leverage for cognitive modeling that SMEs
Analogical Match from Kola to Zerdiathe country (TAS).
Rule pile : uppgiasa5egi-7xExxfiles lumber Of !latch ITpctk.scs : 12 lumbar e! G11xps : 1
Gsap at :
(IIPPIIESS-IUITU ILPPIILS$-61UtCt) (LTTLCZ-tNI1U LTTLCL-GLORLC*) (TLEt-fATIUS guy-$UPUCOMPtfDR)
(VLRLItt-!UtTER VLRLItt-GaURACI) (DUIM-PtUTUhS DESIRE-SUPtlCOMPu?u)
(IAS-rLLTIUS USE-SUPUCOMPUTU) (UM-ft*TIUS 391-SUPflCOMPU1Yt) (VliPOV-IOY VELPOV-80V Leaps: (RLRLat 2UDIL12) (FtaTIUUS SUPUCOMPUTUt4) (CROSS-ROW MISSILES%$) (IUITU2 GLGRLC1t3) Vei$kt : 6 .411572
Analogical Match from Karla to Zerdia the eagle (315).
Rule file : appeasxasc-7atek .salas lumber if Match ITpctkeses : 14 Ember of GMaps : I
G7ap at :
(O?FU-fU-1IUS O?FIR-rtLTIfS) (Tilt-ptaTituS U19-P*LTIUS) (LTTaCZ-UITU TTLC*-SPORTSMLS)
(SLL-ULRLL Sit-ZERDIL) (la$-rUUTIZU IL$-lUTI*RS) tDIRD-SaRLI IRD-LUDIL) (VLLPOI-$0V WEAPON-10W) (DCS U-fLLTURS DLSIRU-fl1TlDRS) (VLRLItt-NITER VaILItt-SPORTSMM) (PUSOI-IVITU PUSOl-sPORT$MM)
%saps : (PLaTIUS3 ?ELTUZSV) (CROSS-SOV4 CROSS-S0V1O) (IUITU2 SPORTSMall) (t&RLat 2UDIL7) Vea6kt : 7 .702646
Fig. 16. SMEs analysis of story set 5. using the MA rules.

THE STRUCTURE-MAPPING ENGINE 39
architecture provides. We know of no other general-purpose matcher which successfully models two distinct kinds of human similarity comparisons . Sec- ond, the short story analogies show that SME is capable of matching large structures as well as the smaller . simpler structures shown previously .
4.5. Testing structure-mapping constraints
While structural consistency is built into the SME algorithm. the other con- straints of structure-mapping are not. This allows us to- emulate a space of matchers and explore extensions and variations of structure-mapping . It also provides a means to determine just how much work the various theoretical
constraints contribute, by modifying or deleting the rules which implement them . Recall that identicality is implemented by the match constructor rules, and systematicity is implemented by match evidence rules . We could determine how much work identicality does . say, by changing the rules to allow arbitrary predicates to match.
We constructed the free-for-all (FFA) rule set to explore just that question. The FFA rule set builds a match hypothesis for every pair of predicates and every pair of entities, regardless of identity, type, or number of arguments. The only constraint still enforced is commutativity-the current implementa-. tion will not allow a commutative predicate to match a noncommutative one Every match hypothesis receives an initial score of 0 .2. Gmap selection is based strictly on systematicity. This rule set has been successfully tested on several graph isomorphisms taken from [541, as well as a few hand-crafted
examples. FFA rules on the It is interesting to examine SSIEs performance with the
simple water flow/heat flow example of Fig . 2. SME constructed 74 match hypotheses, which were combined into 26 gmaps . Four gmaps were tied for the highest ranking (SES = 3.71). Two of these gmaps were supersets of the standard interpretation (i.e.,Gmap #I, where PRESSURE maps to TEMPERA- TURE, as shown in Fig. 7).The new correspondences involved attributes of the beaker-coffee pair. In one gmap. (DIAMETER beaker) was mapped to (LIQUID coffee) and (CLEAR beaker) was mapped to (FLAT-TOP coffee) . and the other reversed these attribute mappings. The other two gmaps were supersets of Gmap #2 from Fig . 7. which paired the diameter inequality from the water
flow base domain to the temperature inequality in the heat flow target domain. The new correspondences in these gmaps included matches from (PRESSURE beaker) and (CLEAR beaker) to (LIQUID coffee) and (FLAT-TOP coffee).
Our experiments with the FFA rules suggest two conclusions. First, while structural consistency is a powerful guide to matching, it is not by itself
sufficient to constrain a matcher . In our simple heat/water analogy, for example, the number of match hypotheses increased by a factor of 5 and the gmaps increased by a factor of 3. Larger examples will provide even worse

.Y eY
N H D O le C
40
B. FALKENHAINER ET AL .
s- v
rt X ^ ` Z ^ T r
n ,v m
E= C0 VE

Z a>K
v sv
E F
C_ C
2
f+1 e+: .C
N Cr y
wOC EdE CJ
ZH
t :h -.
N N N N CIO
H
.? . ~ N Ny N N
I
I
w Ew s
.~ ^ < <I <~<1~w E~~ to h . Hya<< pressure) ((greater (diameter beaker) (diameter vial)) :name >diameter) ((cause >pressure wflow) :name cause-flow)
(flat-top water)
(liquid water)))
B .1.2. Heat flow
(defEntity coffee :type inanimate) (deEntity ice-cube :type inanimate) (defEntiy bar type inanimate) (defEntiy heat :type inanimate)
(defoescviption simple-heat-flow entities (coffee ice-cube bar heat) expressions
(((flow coffee ice-cube heat bar) :name Now) . ((temperature coffee) :name temp-ooffee)
((temperature ice-cube) :name temp-ice-cube)
((greater temp-coffee temOce-cubs) :name >temperature) (flat-top coffee)
(liquid coffee)))
B.2. Solar system/Rutherford atom B .2.1. Solar system
(defEntity sun type inanimate) (defEntiy planet type inanimate)
(defDescription solar-system entities (sun planet)
expressions
(((mass sun) :name mass-sun)
((mass planet) :name mass-planet)
((greater mass-sun mass-planet) :name >mass) ((attracts sun planet) :name attracts) ((revolve-around planet sun) :name revolve)
((and >mass attracts) :name ands)
((cause ands revolve) :name cause-revolve) ((temperature sun) :name temp-sun)
((temperature planet) :name temp-planet)
((greater temp-sun temp-planet) :name >temp) ((gravity mass-sun mass-planet) :name force-gravity) ((cause force-gravity attracts) :name why-attracts)))

THE STRUCTURE-MAPPING ENGINE 57
B.2.2. Rutherford atom
(detEntity nucleus :type inanimate) (detEntity electron type inanimate)
(defDescription rutherford-atom entity (nucleus electron) expressions
(((mass nucleus :name mass-n)
((mass electron) :name mass-e)
((greater mass-n mass-e) :name >mass)
((attracts nucleus electron) :name attracts) ((revolve-around electron nucleus) :name revolve) ((charge electron) :name q-electron)
((charge nucleus) :name q-nucleus)
((opposite-sign q-nucleus q-electron) :name >charge) ((cause >charge attracts) :name why-attracts)))
B .3. Karla stories
B .3.1. Zerdia the eagle: Base story
(defEntity Karla) (defEntiy hunter) (defEntity feathers) (defEntiy cross-bow) (defEntity Failed) (defEntity high)
(defDescription base-5
entities (Karta hunter feathers cross-bow Failed high) expressions
(((bird Karla) :name bird-Karta)
((person hunter) :name person-hunter)
((warlike hunter) :name warike-hunter)
((Karlas-asset feathers) :name feathers-asset)
((weapon cross-bow) :name weapon-bow)
((used-for feathers cross-bow) :name has-feathers)
((not has-feathers) :name not-has-feathers)
((attack hunter Karla) :rearm attack-hunter)
((not attack-hunter) :name not-attack)
((see Karla hunter) :name see-Karla)
((follow see-Karla attack-hunter) :name follow-see-attack) ((success attack-hunter) :name success-attack)
((equals success-attack Failed) :name failed-attack)
((cause not-has-feathers failed-attack) :name cause-failed-attack) ((desire hunter feathers) :name desire-feathers)
((realize Karla desire-feathers) :name realize-desire)
((follow failed-attack realize-desire) :name follow-realize)
((offer Karla feathers hunter) :name offer-feathers)
((cause realize-desire offer-feathers) :name cause-offer) ((obtain hunter feathers) :name take-feathers)
((cause offer-feathers take-feathers) :name cause-take)

58
B. FALKENHAINER ET AL .
((happiness hunter) :name happiness-hunter)
((equals happiness-hunter high) :name happy-hunter)
((cause take-feathers happy-hunter) :name cause-happy) ((promise hunter Karla not-attack) :name promise-hunter) ((cause happy-hunter promise-hunter) :name cause-promise)))
B .3.2. Zerdia the country : TA5
(defEntity Zerdia) (defEntity Gagrach) (defEntity supercomputer) (defEntity missiles) (defEntity failed)
(do fEntity high)
(defDescription to-5
entities (Zerdia Gagrach supercomputer missiles failed high) expressions
(((country Zerdia) :name country-Zerdia)
((country Gagrach) :name country-Gagrach)
((warlike Gagrach) :name warlike-Gagrach)
((Zerdias-asset supercomputer) :name supercomputer-asset) ((weapon missiles) :name weapon-bow)
((use-for supercomputer missiles) :name use-supercomputer) ((not use-supercomputer) :name not-use-supercomputer)
((attack Gagrach Zerdia) :name attack-Gagrach)
((not attack-Gagrach) :name not-attack)
((success attack-Gagrach) :name success-attack)
((equals success-attack failed) :name failed-attack)
((cause not-use-supercomputer failed-attack) :name cause-failed-attack) ((desire Gagrach Supercomputer) :name desire-supercomputer) ((realize Zerdia desire-supercomputer) :name realize-desire)
((follow failed-attack realize-desire) :name follow-realize)
((offer Zerdia supercomputer Gagrach) :name offer-supercomputer) ((cause realize-desire offer-supercomputer) :name cause-offer) ((obtain Gagrach supenowmmputer) :name buy-supercomputer) ((cause offer-supercomputer buy-supercomputer) :name cause-buy) ((happiness Gagrach) :name happiness-Gagrach)
((equals happress-Gagrach high) :name happy-Gagrach)
((cause buy-supercomRxtK happy-Gagrach) :name cause-happy) ((promise Gagrach Zerdla not-attack) :name promise)
((cause happy-Gagrach promise) :name cause-promise)))
B .3.3. Zerdia the hawk:MAS
(defEntity Zerdia) (defEntity sportsman) (deEntity feathers) (defEntity cross-bow) (defEntiy true)
(defDescnption ma-S
entities (Zerdia sportsman feathers cross-bow true)

THE STRUCTURE-MAPPING ENGINE 59
expressions
(((bird Zerdia) :name bird-Zerdia)
((person sportsman) :name person-sportsman)
((warlike sportsman) :name warlike-sportsman) ((Zerdias-asset feathers) :name feathers-asset)
((weapon cross-bow) :name weapon-bow)
((used-for feathers cross-bow) :name has-feathers) ((desire sportsman feathers) :name desire-feathers) ((realize Zerdia desire-feathers) :name realize-desire) ((offer Zerdia feathers sportsman) :name offer-feathers) ((cause realize-desire offer-feathers) :name cause-offer) . . ((obtain sportsman feathers) :name take-feathers)
((cause offer-feathers take-feathers) :name cause-take) ((attack sportsman Zerdia) :name attack-sportsman) ((not attack-sportsman) :name not-attack)
((promise sportsman Zerdia not-attack) :name promise) ((cause take-feathers promise) :name cause-promise) ((see Zerdia sportsman) :name see-Zerdia)
((follow promise see-Zerdia) :name follow-promise)
((follow see-Zerdia attack-sportsman) :name follow-see)
((success attack-sportsman) :name success-attack)
((equals success-attack true) :name successful-attack)
((cause has-feathers successful-attack) :name cause-success-attack) ((realize Zerdia has-feathers) :name realize-Zerdia)
((follow successful-attack realize-Zerdia) :name follow-suck-attack)))
ACKNOWLEDGMENT
The authors wish to thank Janice Skorstad . Danny Bobrow, and Steve Chien for helpful comments on prior drafts of this paper. Janice Skorstad provided invaluable assistance in encoding domain models. Alan Frisch provided pointers into the unification literature .
This research is supported by the Office of Naval Research. contract no. N00014-85-K-0559. Additional support has been provided by IBM . both in the form of a Graduate Fellowship for
Falkenhainer and a Faculty Development award for Forbus . The equipment used in this research was provided by an equipment grant from the Information Sciences Division of the Office of Naval
Research. and by a gift from Texas Instruments.
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