Unit1-Introduction
To introduce what this part of the course is going to cover, let’s pose and look again at the meaning of
the term “Artificial Intelligence”. Intelligence can be defined in many ways and if you look around at
various definition you will see that it surely includes the ability to understand, learn, acquire and use
knowledge to solve given tasks, think abstractly and reason about problems in order to come up with
best solutions and plans, to communicate and to perceive the external environment. Artificial
intelligence is on the other hand the theory and development of computer software (or systems) able to
perform tasks that normally require human intelligence, such as visual perception, speech recognition,
decision-making, and translation between languages. In essence, the study and the development of
intelligence software agents, machines that act intelligently, that manifest some of the capabilities that
the human mind has (e.g., understand language, recognize pictures, solve problems, and learn). The
wide field of Artificial Intelligence encompasses research areas and efforts in each of these different
aspects of intelligence, and for each of these area the focus is on understanding the principles that
underpin such capabilities as well as developing computational mechanisms that enable the realisation
of efficient and effective computational systems that perform tasks commonly viewed as requiring
intelligence.
In this part of the course we will concentrate on some of the fields of Artificial Intelligence listed
above, specifically, on knowledge representation, reasoning, planning and problem solving. Fields such
as learning, robotics, and neural networks will be covered in other courses in the third and four year of
your degree. Some of the aspects that we cover in this course will provide you the foundations for
courses taught in the third and four year that are related to AI. For instance, knowledge representation
and reasoning and planning, will be useful in the logic-based learning and in the systems verification
courses taught in the third year, as well as in the knowledge representation course taught in the forth
year.
Part of the material included in this second part of the course is also based on the textbook “Artificial
Intelligence, foundations of computational agents”, by David Poole and Alan Mackworth.
1
To give you a brief overview of the history of AI, the root of AI started long time ago (around 400 years
ago) when philosopher such as Hobbes, started to study the notion of symbolic reasoning, as symbolic
operations that capture the process of human thinking. In the early 1900, the advent of the Turing
machine contributed further to the notion of symbolic operations, as concrete computational processes.
Different models of computations were developed, among which the model of a Turing machine, an
abstract machine that writes symbols on an infinitely long tape, and the lambda calculus of Church, a
mathematical formalism for rewriting formulas. Once real computers were built (around 1950) some of
the first applications of computers were AI programs, such as a checkers program (i.e. a program that
learns to play checkers – or draughts) and the Logic Theorist, a program that discovers proofs in
propositional logic. Together with high-level symbolic reasoning there was also some emphasis on low-
level learning, which was initially inspired by the perceptron formal neuron model developed by
Rosenblatt in 1958.
All these examples of programs focused mainly on learning and search problems. It became then
apparent that to provide intelligent solutions to problems it was essential to tackle the main problem of
how to represent the knowledge needed to solve a given problem. In the 80s, AI research activity saw
the proliferation of many proposals from simple to more complex knowledge representation and
languages based on logic (see McCarthy and Hayes), which then naturally led to first systems able to
understand simple, controlled natural language interactions. Early examples were Chat-80, which was a
Q&A system designed to answer geographical questions. Expert systems then followed, where the aim
was to capture a large body of knowledge of an expert in some domain so that a computer could carry
out expert tasks. At the same time further work was done in AI in the area of automated reasoning and
languages, and systems such as Prolog became established in the AI community. From 1990 onwards,
specific sub-disciplines of AI such as perception, probabilistic and decision-theoretic reasoning,
planning, machine learning, and many other fields started to grow.
Reasoning and learning have been since then at the center of AI research. You will see more about
learning in subsequent courses in your third and four year. In the rest of this course we concentrate on
the reasoning and knowledge representation and problem solving aspects of AI.
2
We have mentioned at the beginning that AI is, in essence, the study and the development of
intelligence computational agents. What we aim to cover in this course is the study of principles that
underlie intelligent computational agents. First of all, we should define what an intelligent
computational agents is.Looking at the definition of David Pool in his text book on “Artificial
Intelligent, foundations of computational agents”, an intelligent agent is an agent that acts in an
environment. At any time, it decides what actions to perform, based on observations of the current
environment, prior knowledge about the environment and task that it has to accomplish, goals that it
must try to achieve, also with preferences over states of the world, and of course its own abilities,
that is primitive actions that it is capable of carry out. In an intelligent agent, decisions may also be
informed by past experience, that is knowledge about previous actions and observations of effects
these actions in given states of the environment. Past experience can be used by the agent as new
learned knowledge to improve the execution of its task. Prior knowledge is, in general, knowledge
that cannot be learned through experience.
We can have different types of agents. An agent could, for example, incorporates a computational
engine with physical sensors and actuators and act in an environment which is a physical setting.
This is the case of a robot agent. It could be an advice-giving computer – a digital assistant– where
the environment the agent has to interact with is the human. The human provides the perceptual
information and carries out the task. Or, we could have a software agent. This is an agent that acts in
a purely computational environment (e.g., smart meter controller).
3
Consider for instance the case of a robot agent that has to perform the task of carrying a ball
from one place to another in an room environment (goal). Its observations are success and
failure of its moving actions. Past experiences are logs of previous actions performed in given
states of the worlds and success and failure of those actions. Its prior knowledge is the effects of
each of its actions, and abilities are the actions of “pickup” an object, “move” and “putdown” an
object.
The agent in this case is intelligent because it is able to reason about the environment and
construct plans, but also learn better models of its environment based on observations and past
experience and use this knowledge to decide what is the best action to perform at each current
time point given the state in which the environment and the robot are. Past experience and
observations, as the environment changes, provide new evidence to the robot, which it then uses
to learn new knowledge about the environment.
4
5
6
This is just an example of how a prior (or domain) knowledge can be represented in the case of the
robot agent moving in an environment. You don’t need to know this representation of understand the
full details here. The important message is the fact that the robot collects logs of executions over
time some of which lead to successful task and some of which do not (as for example moving to a
position 3 in the environment where the object breaks). The knowledge learned about what action is
successful under what conditions and what is the likelihood of each of these learned rules.
The probabilities are learned so explain as much as possible the collected observations.
7
The learned knowledge may represent a new model of the environments and
actions to perform in this environment.
8
This is an example of knowledge representation and AI solutions such as symbolic machine
learning, could be used to allow a robot agent to sustain human-machine dialogue, and to learn
common-sense knowledge or domain specific knowledge through dialogue.
This work was done as student project by an MSc student in summer 2016.
9
This slide describes the general framework for solving problems by computer. To solve a problem,
we must first define the task and determine what constitutes a solution. This is normally done
informally (see top part of the diagram above). We need then to represent the problem in a language
with which a computer can reason. Use the computer to compute an output. This can be an answer,
which can be presented to a user (as it is the case of a digital assistant) or a sequence of actions to be
carried out in the environment ( as it is the case for a robot agent). The output has then to be
interpreted as a solution to the problem.
Knowledge is the information needed to solve a problem in a given domain. So, as part of the design
(or modelling) of a computational agent capable of solving a problem, we must define how the
knowledge will be represented in the agent. There are different forms of representation of knowledge.
These are referred to as knowledge schemas. You have, for instance, seen in the first part of the
course the logic programming formalism. This is an example of representation schema, whereby
knowledge (or information) is stored or represented into an agent as set of clauses or Prolog rules.
Many different forms of representation of knowledge can be used, e.g.symbolic, probabilistic,
statistical, etc. Each of these forms may be more suited to certain tasks and not others. For instance,
in domains of “certainty”,a symbolic, logic-based form of representation of knowledge is
appropriate. Different are the cases of approximate domains and uncertainty. Probabilistic forms of
representation of knowledge might be more suitable. Once we fix a representation schema, then the
formalisation of a piece of knowledge in that schema is called representation of the knowledge.
Different schemas give rise to different representations of the knowledge related to a given problem.
The representation of the entire knowledge that is stored in an agent is instead referred to as
knowledge base of the agent.
10
A representation should be
• rich enough to express the knowledge needed to solve the problem. In the first part of this course, you
have seen logic programming as a symbolic approach for representing knowledge. This is a reasonably
expressing language where, for instance, common sense notions (typically used in digital assistant),
default assumptions, used in all types of agents, could be expressed by means of negation as failure. In
this second part of the course, we will look a much more expressive language, called Answer Set
Programming (ASP), that provides a truly declarative approach for representing knowledge, hard
constraints, weak constraints, and optimisation statements.
• close to the problem. Declarative formalisms such as logic programming, ASP and logic in general, are
as close to a problem description as possible. ASP in particular is known to be more declarative than
logic programming as it does not mix in its representation style procedural aspects for solving a
problem, as well as declarative aspects for defining the problem. It is purely descriptive and can be
used to describe a problem solution in terms of its properties and specification. Because of this, ASP is
also easy to relate to a given problem domain than Prolog representations. It is also very compact in its
representation and natural. We will see later on why this is the case.
• amenable to efficient computation, which usually means that it is able to express features of the
problem that can be exploited for computational gain and able to trade off accuracy and computation
time. Depending on the type of formalisms, accuracy of solutions may or may not be something that
can be trade. For instance, in logic programming, the representation is underpinnedby a precise
semantic and solutions, when they exist, are always guaranteed to be 100% precise. In the case of ASP,
instead, optimisation statements can be used to generate optimal and suboptimal solutions.
Computation time can be traded off with accuracy. Fast computations may look for sub-optimal
solutions instead of“the” optimal solutions.
• able to be acquired from people, data and past experiences. This feature refers to the ability of an agent
to learn knowledge automatically. In the logic-based learning course, which is taught in the third year,
you will be able to see how you can automatically learn knowledge represented as ASP programs from
given data and past experiences.
11
Before representing the knowledge that is needed to solve a problem in a given domain, it is
important to define what would constitute a solution of the given problem. Typically, there are four
common classes of solutions:
Optimal Solutions: this are the ones that are the best solutions according to some criteria for
measuring the quality of solutions. These criteria can be qualitative (also said ordinal) measures
(e.g. a robot that takes out “as much trash” as possible, the more trash the better is the solution), or
quantitative (also said cardinal) measures (e.g. a delivery robot, has to deliver as many packets but
minimizing the distance travelled.
Satisficing solutions: sometime it is not necessary to find the best optimal solution but just some
solution. A satisficing solution is one that is good enough according to some notion of what is an
adequate solution. For example, a person may tell a robot that it must take all of trash out, or tell it
to take out three items of trash. Or, in the case of a natural language interactions with digital
personal assistant, satisficing solutions are those systems that are able to answer most of the
questions in a satisficing manner.
Approximately optimal solution: these are solutions that are close enough to optimal solutions.
Agents may not need optimal solutions to problems; they only need to get close enough. For
example, a delivery robot may not need to travel the optimal distance to take out the trash but may
only need to be within, say, 10% of the optimal distance.
Probable solution: A probable solution is one that, even though it may not actually be a solution to
the problem, is likely to be a solution. This is one way to approximate, in a precise manner, a
satisficing solution. For example, in the case where the delivery robot could drop the trash or fail
to pick it up when it attempts to, we may need the robot to be 80% sure that it has picked up some
items of trash.
12
Once we have an idea of the nature of a solution, we can choose the level of representation of the
problem so that a machine can solve the problem and compute the type of required solutions. The
representation of the problem constitutes essentially a model of the world in which an intelligent
agent has to operate to solve its task.
So, we can define a model to be a representation of the specifics of what is true and false in the world.
The world does not have to be modeled at the most detailed level to be useful. All models are
abstractions; they represent only part of the world and leave out many of the details. An agent can
have a very simplistic model of the world, or it can have a very detailed model of the world. A lower-
level abstraction includes more details than a higher-level abstraction. An agent can have multiple,
even contradictory, models of the world (see argumentation models in the four year course).
In general, there isn’t a best level of abstraction, and the best way to judge whether a model is a good
representation is if it is useful to solve a given task.
13
Choosing an appropriate level of abstraction is difficult:
• high-level description, such the natural language example you have seen in the short video, as a form
of explanation that a system can be give to a human, is easier for a human to understand the system’s
solution and also easier for a human to describe the problem and give feedback. But it is harder for a
machine to fully comprehend due to the high-level of ambiguity and also the common-sense knowledge
that is often implicitly assumed by humans when they communicate in NL. Despite the tremendous
progress witnessed so far in NLP , we are still far for representing a problem in NL directly, as
machines are still not able to rigorously reason directly with NL.
• a low-level description can be more accurate and more predictive. Often high-level descriptions
abstract away details that may be important for actually solving the problem. So certain type of
problems can be solved only if detailed models are given, but
• the lower the level, the more difficult it is to reason with. This is because a solution at a lower level of
detail involves more steps and many more possible courses of action exist from which to choose.
• you may not know the information needed for a low-level description as it is too detailed and
environment changes at micro-level more than at abstract level. Consider for instance the GPS location
of a phone. At a low-level representation of the problem, GPS data change very frequently whereas at a
higher-level of representation the state property of a human walking persists. So reasoning about human
behaviors at low-level of representation is clearly harder that reasoning using the more abstract
representation of similar concepts (e.g. person walking).
The example given in this slide shows different level of abstraction in the context of learning (privacy)
policies about human behaviors with mobile phones. On the left, data are collected by sensors. The
low-level representation is quite detailed, imagine for instance GPS coordinates versus GPS names. But
all these data can be used by an intelligent agent to learn user behaviors from contextual low-level data
information and user’s past actions. The outcome is high-level representation of policies that the system
can communicate back to the user for validation purposes.These policies are then used by the system
to self-adapt (at run-time). Models in these cases can have a mix level of representation. It is often a
good idea to model a given problem at multiple levels of abstraction.
14
The term “reasoning” is used to denote the process made by a computational agent when searching
for possible ways to determine how to complete its task. Depending of the tasks, for instance, these
possible ways can be actions, as it is in the case of a delivery robot that has to choose the next
physical action needed to complete its task, or computing (diagnostic) answers, as it is in the case
of a diagnostic assistant that has to compute possible diagnosis for some given symptoms.
The reasoning process, in general, can be performed by a computational agent either “offline” or
“online”.
Offline reasoning assumes that the background knowledge needed to solve the task is
precomputed. Agents receive, at design time, this knowledge and (environment) data and perform
reasoning to solve its task. This is for instance the case of computational agents such as a
manufacturing robots. The knowledge is all precompiled so that the agent can perform simple
offline reasoning and perform its task in a predefined constrained environment. Examples are car
painting robots. They are very specialized agents that do not adapt well to different environments
or to changing tasks.
Online reasoning, instead, allow agents to “sense” the environment in which they operate so to
collect online data during the execution and use these data, together with the given background
knowledge, to decide at execution time what action to take. The reasoning in this case is performed
between the sensing of environment data and the action. This is for instance the case ofboth a
delivery robot or a diagnostic assistant. In the latter case the environment would be the human who
provide information to the systems as new sensed data.
In general, a reasoning process can be of different types: deductive, abductive and inductive. In
this course we will cover the first two types of reasoning in detail. Inductive reasoning will instead
be covered in the 3rd year course on “Logic-based learning”.
15
When designing a computational agent, we have to make decisions on different aspects of the system that can
affect the level of computational complexity of the agent.
In this slide we highlight some of the key aspects that we will need to take into account when we develop an
AI solution to a given problem:
1) Model of the environment. These can be expressed in terms of states, features and/or individuals and
relations between individuals. For example, in the robot agent shown in slide 4, the model of the
environment is expressed in terms of states and the agent reasons about the states the environment can be
in when it has to decide which action to take. In the case of a diagnostic assistant, it might be more
appropriate to consider features and features values instead of individual states, as they may be too many.
2) Uncertainty. This can be of two types, uncertainty on the state and uncertainty of effects of actions. In the
first case, perception of the state in which the environment is maybe “certain” and the agent known
exactly in which state the environment is (we call this full observable state), or the perception of the state
the environment may be noisy, in which case that can be more than one possible state of the environment
that can lead to the same agent’s perception (we call this partially observable state). For instance, in the
case of a diagnostic assistant, the agent cannot really sense the disease of the patient, (patient is in this
case the environment and the diseases possible states), but multiple diseases may give the same
symptoms. So the agent can only reason with the set of possible diseases. Uncertainty may also be on
effects of actions. Effects of actions can be deterministic (next state uniquely determined by the current
state and the action) or effects maybe stochastic (when we have possible next states and the agent knows
some probability distribution over the next states). This is the case in slides 7 and 8.
3) Preferences. These occur when there are trade-off between the desirability of possible outcomes. For
instance, in the robot agent it is more important to carry the item without breaking than to take the shortest
path in the environment, as this might lead to states where there is a risk of dropping the item. They can be
ordinal, when we need just a ranking or ordering among goals or actions;and cardinal, when the
magnitude of the values matters (e.g. costs).
4) Number of agents. Some problem need to be solve by an agent taking into account also the
reasoning/acting of other agents in the environment. The other agents may act to trick or maybe willing to
16
cooperate. You have seen examples in the first part of this course.
16
17
We have seen that from an AI point of view, representation of a problem and reasoning are key elements.
In the first part of this course, you have covered different forms of representation considering in particular
logic programming, as a main formalism for representing a problem domain and an agent’s knowledge. You
have also covered different semantics and looked at the deductive inference mechanisms typical for Prolog
(i.e. SLD and SLDNF). These are normally goal-directed inference systems.
In this second part of this course we will look at a different form of inference, called abductive inference.
This is typically used to solve AI problems such as diagnosis (as it is the case for a diagnostic assistant
agent), explanation generation (which is at the core of the current “Explainable AI” trend) and off-line
planning. Abductive reasoning can be computed in different ways depending on the type representation
used to express the problem. We will cover two ways for computing abductive reasoning, a top-down
approach that uses logic programming as representation formalism, and a bottom-up approach that uses
Answer Set Programming.
We will also in particular cover Answer Set Programming (ASP), considered to be currently one of the
most efficient declarative programming languages, also particularly suited for representing and solving AI
problems.ASP uses a different semantics and a different reasoning process for computing solutions of a
given problem. The reasoning process of ASP builds upon another traditional AI task, called SAT solving.
So, we will also cover basic notions of SAT solving and explore more expressing mechanisms for
representing and solving a SAT problem, including related algorithms. The formalisms covered in this case
with be closer to full classical logic, than logic programming.
The aims of this second part of the course are:
§ learn bottom-up formalism and semantics for representing knowledge in a computational agents
§ learn foundation principles of planning and abductive reasoning
§ familiarize with existing solvers such as ASP solver and SAT solvers.
§ use solvers to solve planning problems.
At the end of this course you will have acquired fundamental principles, technologies in the AI areas of
planning, knowledge, perception and cognitive computing, which will provide you with the essential
building blocks for the AI courses in your third and forth year.
18
The first textbook is a good introduction to logic programming and basic notions of Prolog that we will be
referring to during this course.
The second textbook is one of the main textbooks of this course. Notes on this lecture as well as other
lectures to come are based on this textbook. So it’s good read if you want to extend your knowledge on the
topics covered in this course.
In the third reference is the link to ASP, where you will find the Clingo system and relatedmanual. You
will need to familiarise with the manual and system available on this Website as you will be doing some
exercises and tutorials using Clingo. Note that the material is available on a Potsdam university’s link,
because a research group at thisuniversity has developed the Clingo system, which is now one of the
most used programming environments for solving problems written in Answer Set Programming.
The fourth reference is another general textbook on knowledge representation. Some of these topics will
covered in more detailed in the knowledge representation course in the forth year.
I will also suggest papers as additional reading if and where needed, during the course.
The above list of topics is not meant to follow any chronological order in which we will be
covering them.
19
Reviews
There are no reviews yet.