Transcript CS344 : Introduction to Artificial Intelligence Pushpak Bhattacharyya CSE Dept., IIT Bombay Lecture 4- Logic.

CS344 : Introduction to Artificial
Intelligence
Pushpak Bhattacharyya
CSE Dept.,
IIT Bombay
Lecture 4- Logic
Logic and inferencing
Vision
NLP
Search
 Reasoning
 Learning
 Knowledge

Robotics
Expert
Systems
Planning
Obtaining implication of given facts and rules -- Hallmark of
intelligence
Inferencing through
−
−
−
Deduction (General to specific)
Induction (Specific to General)
Abduction (Conclusion to hypothesis in absence of any other evidence
to contrary)
Deduction
Given:
All men are mortal (rule)
Shakespeare is a man (fact)
To prove:
Shakespeare is mortal (inference)
Induction
Given:
Shakespeare is mortal
Newton is mortal
(Observation)
Dijkstra is mortal
To prove:
All men are mortal (Generalization)
If there is rain, then there will be no picnic
Deduction
Fact1: There was rain
Conclude: There was no picnic
Fact2: There was no picnic
Conclude: There was no rain (?)
Induction and abduction are fallible forms of reasoning. Their conclusions are
susceptible to retraction
Two systems of logic
1) Propositional calculus
2) Predicate calculus
Propositions
Stand for facts/assertions
− Declarative statements
− As opposed to interrogative statements (questions) or imperative
statements (request, order)
−
Operators
AND
(

),
OR
(

),
NOT
(~),
IMPLIC
N
(

)
=> and ¬ form a minimal set (can express other operations)
- Prove it.
Tautologies are formulae whose truth value is always T, whatever the
assignment is
Model
In propositional calculus any formula with n propositions has 2n models
(assignments)
- Tautologies evaluate to T in all models.
Examples:
1)
P  P
2)
( P  Q)  (P  Q)
e Morgan with AND
-
Semantic Tree/Tableau method of proving tautology
Start with the negation of the formula
[( P  Q)  (P  Q)]
- α - formula
α-formula
β-formula - β - formula
( P  Q)
α-formula
(P  Q)
- α - formula
p
q
¬p
¬q
Example 2:
[ A  ( B  C )  ( A  B)  ( A  C )]
X
(α - formula)
A  (B  C)
(α - formulae)
¬A
(( A  B)  ( A  C )) α-formula
( A  B)
¬A
¬C
¬B
( A  C ))
¬B
¬A
(β - formulae)
A
A
B∨ C
A
B∨ C
B∨ C
Contradictions in all paths
B
A
C
B∨ C
B
C
Exercise:
Prove the backward implication in the previous
example
Formal Systems
Rule governed
 Strict description of structure and rule application


Constituents


Symbols
Well formed formulae

Inference rules

Assignment of semantics

Notion of proof

Notion of soundness, completeness, consistency,
decidability etc.
Hilbert's formalization of propositional calculus
1. Elements are propositions : Capital letters
2. Operator is only one :

(called implies)
3. Special symbol F (called 'false')
4. Two other symbols : '(' and ')'
5. Well formed formula is constructed according to the grammar
WFF P|F|WFFWFF
6. Inference rule : only one
AB and
Given
A
write B
known as MODUS PONENS
7. Axioms : Starting structures
( A  ( B  A))
A1:
A2:
(( A  ( B  C ))  (( A  B)  ( A  C )))
A3
(((A  F )  F )  A)
This formal system defines the propositional calculus
Notion of proof
1. Sequence of well formed formulae
2. Start with a set of hypotheses
3. The expression to be proved should be the last line in the
sequence
4. Each intermediate expression is either one of the hypotheses
or one of the axioms or the result of modus ponens
5. An expression which is proved only from the axioms and
inference rules is called a THEOREM within the system
Example of proof
From P and P  Q
and Q  R prove R
H1: P
H2: P  Q
H3: Q  R
i) P
H1
ii) P  Q
H2
iii) Q
MP, (i), (ii)
iv) Q  R
H3
v) R
MP, (iii), (iv)
Prove that ( P  P) is a THEOREM
i) P  ( P  P)
A1 : P for A and B
ii) P  ((P  P)  P)
A1: P for A and ( P  P) for B
iii) [(P  ((P  P)  P))  ((P  ( P  P))  ( P  P))]
A2: with P for A,
( P for
P) B and P for C
iv)( P  ( P  P)  ( P  P))
MP, (ii), (iii)
v) ( P  P)
MP, (i), (iv)
Formalization of propositional logic (review)
Axioms :
A1
( A  ( B  A))
(( A  ( B  C ))  (( A  B)  ( A  C )))
A2
(((A  F )  F )  A)
A3
Inference rule:
Given ( A  B) and A, write B
A Proof is:
A sequence of
i) Hypotheses
ii) Axioms
iii) Results of MP
A Theorem is an
Expression proved from axioms and inference rules
Example: To prove ( P  P)
i) P  ( P  P)
A1 : P for A and B
ii) P  ((P  P)  P)
A1: P for A and ( P  P) for B
iii) [(P  ((P  P)  P))  ((P  ( P  P))  ( P  P))]
A2: with P for A,
( P for
P) B and P for C
iv)( P  ( P  P)  ( P  P))
MP, (ii), (iii)
v) ( P  P)
MP, (i), (iv)
Shorthand
1. ¬ P
2.
is written as
((P  F )  Q)
PF
and called 'NOT P'
is written as ( P  Q) and called
'P OR Q’
3.
((P  (Q  F ))  F ) is
written as
'P AND Q'
Exercise: (Challenge)
- Prove that
A  (( A))
( P  Q)
and called
A very useful theorem (Actually a meta
theorem, called deduction theorem)
Statement
If
A1, A2, A3 ............. An ├ B
then
A1, A2, A3, ...............An-1├
An  B
├ is read as 'derives'
Given
A1
A2
A3
.
.
.
.
An
B
A1
A2
A3
.
.
.
.
An-1
Picture 1
An  B
Picture 2
Use of Deduction Theorem
Prove
A  (( A))
i.e.,
A  (( A  F )  F )
A, A ├FF
A├ ( A  F )  F
├
A  (( A  F )  F )
(M.P)
(D.T)
(D.T)
Very difficult to prove from first principles, i.e., using axioms and
inference rules only
Prove P  ( P  Q)
i.e. P  ((P  F )  Q)
P, P  F , Q  F ├ F
P, P ├F
├Q
P├
├
(Q  F )(D.T)
F
(M.P with A3)
(P  F )  Q
P  ((P  F )  Q)