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Logic
Planning
Logic
Planning
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Formal notation
Conjunctions, disjunctions, negations, implications
Truth tables
Rules of inference
Resolution theorem proving
Why do we need formal logic?
Soundness: Only valid conclusions can be proven.
Completeness: All valid conclusions can be proven.
If an animal has feathers, then it is a bird
If an animal lays eggs and it flies, then it is a bird
Vertebrate
Bird
Bluebird
Penguin
Eagle
Brick
Block
Block
A Foo
Brick
touches
touches
supports
supports
supports
supports
Mark the sufficient conditions:
If
A brick supports two bricks
A brick supports two blocks
Those two blocks touch
Those two blocks do not touch
Those two blocks support a block
Those two blocks support a brick
then the object is a foo
Predicate:
A function that maps object arguments to true or false values
Feathers(bluebird)
Predicate:
A function that maps object arguments to true or false values
If an animal has feathers, then it is a bird
Feathers(animal)
If Feathers(animal):
Then Bird(animal)
If an animal lays eggs and it flies, then it is a bird
Lays-eggs(animal)
Flies(animal)
If Lays-eggs(animal)
Flies(animal):
Then Bird(animal)
If an animal lays eggs or it flies, then it is a bird
Lays-eggs(animal)
Flies(animal)
If Lays-eggs(animal)
Flies(animal):
Then Bird(animal)
Flies(animal)
Bird(animal)
If Flies(animal) Bird(animal):
Then Bat(Animal)
If an animal flies and is not a bird, it is a bat.
If Lays-eggs(animal) Flies(animal):
Then Bird(animal)
Lays-eggs(animal) Flies(animal) Bird(animal)
OperatorSymbolAccepted Symbol
ANDA BA & B
A && B
ORA BA | B
A || B
NOTA!A
~A
IMPLIESA BA = B
A == B
A => B
If an animal lays eggs and does not have feathers, it is a reptile.
Lays-eggs(animal) Feathers(animal) Reptile(animal)
If an animal has feathers or has talons, it is a bird.
Feathers(animal) Talons(animal) Bird(animal)
If an animal lays eggs, has a beak, and flies, it is a duck.
Lays-eggs(animal) Beak(animal) Flies(animal) Duck(animal)
If an animal lays eggs, has a beak, and do not fly, it is a platypus.
Lays-eggs(animal) Beak(animal) Flies(animal) Platypus(animal)
ABA B
TrueTrueTrue
TrueFalseTrue
FalseTrueTrue
FalseFalseFalse
ABA B
TrueTrueTrue
TrueFalseTrue
FalseTrueFalse
FalseFalseTrue
ABA B
TrueTrueFalse
TrueFalseFalse
FalseTrueFalse
FalseFalseTrue
ABCA (B C)
TrueTrueTrueTrue
TrueTrueFalseTrue
TrueFalseTrueTrue
TrueFalseFalseTrue
FalseTrueTrueFalse
FalseTrueFalseTrue
FalseFalseTrueFalse
FalseFalseFalseFalse
ABA BB A
TrueTrueTrueTrue
TrueFalseFalseFalse
FalseTrueFalseFalse
FalseFalseFalseFalse
Commutative Property
Distributive Property
ABCA (B C)(A B) (A C)
TrueTrueTrueTrueTrue
TrueTrueFalseTrueTrue
TrueFalseTrueTrueTrue
TrueFalseFalseFalseFalse
FalseTrueTrueFalseFalse
FalseTrueFalseFalseFalse
FalseFalseTrueFalseFalse
FalseFalseFalseFalseFalse
Associative Property
ABCA (B C)(A B) C
TrueTrueTrueTrueTrue
TrueTrueFalseTrueTrue
TrueFalseTrueTrueTrue
TrueFalseFalseTrueTrue
FalseTrueTrueTrueTrue
FalseTrueFalseTrueTrue
FalseFalseTrueTrueTrue
FalseFalseFalseFalseFalse
de Morgans Law
AB(A B)A B
TrueTrueFalseFalse
TrueFalseTrueTrue
FalseTrueTrueTrue
FalseFalseTrueTrue
Truth of Implications
ABA B
TrueTrueTrue
TrueFalseFalse
FalseTrueTrue
FalseFalseTrue
Implication Elimination
Given:
a b
Rewrite as:
a b
Given:
Feathers Bird
Rewrite as:
Feathers Bird
Modus Ponens
Sentence 1: p q
Sentence 2: p
Sentence 3: q
Feathers Bird
Feathers
Bird
Modus Tollens
Sentence 1: p q
Sentence 2: q
Sentence 3: p
Feathers Bird
Bird
Feathers
Rules of Inference: Instantiate general rules to prove specific claims.
Prove: Harry is a bird
By Modus Ponens
S1: Feathers(animal) Bird(animal)
S2: Feathers(Harry)
S3: Bird(Harry)
Prove: Harry is a bird
S1: Feathers(animal) Bird(animal)
S2: Feathers(Harry)
S3: Bird(Harry)
Prove: Buzz does not have feathers
S1: Feathers(animal) Bird(animal)
S2: Bird(Buzz)
S3: Feathers(Buzz)
By Modus Tollens
Prove: Buzz does not have feathers
S1: Feathers(animal) Bird(animal)
S2: Bird(Buzz)
S3: Feathers(Buzz)
For one animal:
Lays-eggs(animal) Flies(animal) Bird(animal)
For all animals:
x[Lays-eggs(x) Flies(x) Bird(x)]
Universal Quantifier
For one animal:
Lays-eggs(animal) Flies(animal) Bird(animal)
For at least one animal:
y[Lays-eggs(y) Flies(y) Bird(y)]
Existential Quantifier
We know:
S1: can-move liftable
We find:
S2: can-move
How do we prove the box is not liftable?
We know:
S1: can-move liftable
By implication elimination:
S1: can-move liftable
We find:
S2: can-move
We assume:
S3: liftable
How do we prove the box is not liftable?
How do we prove the box is not liftable?
S1: can-move liftable
S2: can-move
S3: liftable
How do we prove the box is not liftable?
S1: can-move liftable
S2: can-move
S3: liftable
How do we prove the box is not liftable?
S1: can-move liftable
S2: can-move
S3: liftable
How do we prove the box is not liftable?
S1: can-move liftable
S2: can-move
S3: liftable
How do we prove the box is not liftable?
S1: can-move liftable
S2: can-move
S3: liftable
We know:
S1: can-move battery-full liftable
We find:
S2: can-move
S3: battery-full
How do we prove the box is not liftable?
We know:
S1: can-move battery-full liftable
By implication elimination:
S1: (can-move battery-full) liftable
We find:
S2: can-move
S3: battery-full
How do we prove the box is not liftable?
We know:
S1: can-move battery-full liftable
By implication elimination:
S1: (can-move battery-full) liftable
By deMorgans Law:
S1: can-move battery-full liftable
We find:
S2: can-move
S3: battery-full
How do we prove the box is not liftable?
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove the box is not liftable?
S1: can-move liftable battery-full
S2: can-move
S3: battery-full
S4: liftable
How do we prove this is a bird?
If an animal has wings and does not have fur, it is a bird.
Write in formal logic:
has-wings has-fur bird
(Use the predicates has-wings, has-fur, and bird)
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
49
How do we prove this is a bird?
has-wings has-fur bird
Use implication elimination to rewrite as a conditional:
(has-wings has-fur) bird
(Use the predicates has-wings, has-fur, and bird)
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
50
How do we prove this is a bird?
(has-wings has-fur) bird
Use de Morgans Law to rewrite in conjunctive normal form:
has-wings has-fur bird
(Use the predicates has-wings, has-fur, and bird)
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
51
How do we prove this is a bird?
S1:
has-wings has-fur bird
S2: has-wings
S3: has-fur
What sentence would be assumed to facilitate the proof?
S4: bird
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
52
How do we prove this is a bird?
S1:
has-wings has-fur bird
S2: has-wings
S3: has-fur
S4: bird
What part of S1 would we eliminate first?
has-wings
has-fur
bird
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
53
How do we prove this is a bird?
S1:
has-wings has-fur bird
S2: has-wings
S3: has-fur
S4: bird
What part of S1 would we eliminate first?
has-wings
has-fur
bird
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
54
How do we prove this is a bird?
S1:
has-wings has-fur bird
S2: has-wings
S3: has-fur
S4: bird
What do we do next?
Resolve on S2 and has-wings from S1
Resolve on S2 and has-fur from S1
Resolve on S3 and has-wings from S1
Resolve on S3 and has-fur from S1
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
55
Assignment
How would you represent Ravens progressive matrices using formal logic?
To recap
Formal notation
Properties of truth values
Rules of inference
Proof by refutation
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