CS 416
Artificial Intelligence
Lecture 11
Logical Agents
Chapter 7
Midterm Exam
Midterm will be on Thursday, March 13th
It will cover material up until Feb 27th
Reasoning w/ propositional logic
Remember what we’ve developed so far
• Logical sentences
• And, or, not, implies (entailment), iff (equivalence)
• Syntax vs. semantics
• Truth tables
• Satisfiability, proof by contradiction
Logical Equivalences
Know these equivalences
Reasoning w/ propositional logic
Inference Rules
• Modus Ponens:
– Whenever sentences of form a => b and a are given
the sentence b can be inferred
 R1: Green => Martian
 R2: Green
 Inferred: Martian
Reasoning w/ propositional logic
Inference Rules
• And-Elimination
– Any of conjuncts can be inferred
 R1: Martian ^ Green
 Inferred: Martian
 Inferrred: Green
Use truth tables if you want to confirm inference
rules
Example of a proof
P?
~P
~B
P?
B
P? P?
Example of a proof
P?
~P
~B
~P
B
~P P?
Constructing a proof
Proving is like searching
• Find sequence of logical inference rules that lead to desired
result
• Note the explosion of propositions
– Good proof methods ignore the countless irrelevant
propositions
Monotonicity of knowledge base
Knowledge base can only get larger
• Adding new sentences to knowledge base can only make it get larger
– If (KB entails a)
 ((KB ^ b) entails a)
• This is important when constructing proofs
– A logical conclusion drawn at one point cannot be invalidated by a
subsequent entailment
How many inferences?
Previous example relied on application of inference
rules to generate new sentences
• When have you drawn enough inferences to prove something?
– Too many make search process take longer
– Too few halt logical progression and make proof process
incomplete
Resolution
Unit Resolution Inference Rule
• If m and li are
complementary
literals
Resolution Inference Rule
Also works with clauses
But make sure each literal appears only once
Resolution and completeness
Any complete search algorithm, applying only the
resolution rule, can derive any conclusion
entailed by any knowledge base in propositional
logic
• More specifically, refutation completeness
– Able to confirm or refute any sentence
– Unable to enumerate all true sentences
What about “and” clauses?
Resolution only applies to “or” clauses
• Every sentence of propositional logic can be transformed to a
logically equivalent conjunction of disjunctions of literals
Conjunctive Normal Form (CNF)
• A sentence expressed as conjunction of disjunction of literals
– k-CNF: exactly k literals per clause
CNF
An algorithm for resolution
We wish to prove KB entails a
• Must show (KB ^ ~a) is unsatisfiable
– No possible way for KB to entail (not a)
– Proof by contradiction
An algorithm for resolution
Algorithm
• KB ^ ~a is put in CNF
• Each pair with complementary literals is resolved to produce
new clause which is added to KB (if novel)
– Cease if no new clauses to add (~a is not entailed)
– Cease if resolution rule derives empty clause (~a is
entailed)
Example of resolution
Proof that there is not a pit in P1,2: ~P1,2
• KB ^ P1,2 leads to empty clause
• Therefore ~P1,2 is true
Formal Algorithm
Horn Clauses
Horn Clause
• Disjunction of literals with at most one is positive
– (~a V ~b V ~c V d)
– (~a V b V c V ~d) Not a Horn Clause
Horn Clauses
Can be written as a special implication
• (~a V ~b V c)  (a ^ b) => c
– (~a V ~b V c) == (~(a ^ b) V c) … de Morgan
– (~(a ^ b) V c) == ((a ^ b) => c
… implication elimination
Horn Clauses
Permit straightforward inference determination
• Forward chaining
• Backward chaining
Horn Clauses
Permit determination of entailment in linear time
(in order of knowledge base size)
Forward Chaining
Forward Chaining
Properties
• Sound
• Complete
– All entailed atomic sentence will be derived
Data Driven
• Start with what we know
• Derive new info until we discover what we want
Backward Chaining
Start with what you want to know, a query (q)
Look for implications that conclude q
Goal-Directed Reasoning