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Logic
Planning
Logic
Planning
Lesson Preview
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
NOT¬A!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
AB¬A ∧ ¬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 Morgan’s 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 deMorgan’s 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 Morgan’s 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?
ο ¬has-wings
ο has-fur
ο 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?
ο ¬has-wings
ο has-fur
ο 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?
ο Resolve on S2 and ¬has-wings from S1
ο Resolve on S2 and has-fur from S1
ο Resolve on S3 and ¬has-wings from S1
ο Resolve on S3 and has-fur from S1
Carl Chapman, http://www.flickr.com/photos/12138336@N02/1997128094/
55
Assignment
How would you represent Raven’s progressive matrices using formal logic?
To recap…
Formal notation
Properties of truth values
Rules of inference
Proof by refutation
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