1 Introduction to Artificial Intelligence LECTURE 9: Resolution in FOL Theorem Proving in First Order Logic Unification Resolution

2 FOL Decidability (1) If a proof procedure is always guaranteed to find a proof of a particular sentence or of its negation, then the question of logical implication for the sentence is said to be decidable. To see if KB |= c, we generate all possible inferences from , stopping when we get  or ~c.

3 FOL Decidability (2) In general, neither  nor ~c may be logically implied by KB. In this case, the proof procedure will never stop. The question of logical implication in this case is thus semidecidable: If KB |= c or KB |= ~c, the proof procedure will eventually discover that. Otherwise, the procedure will run forever.

4 Resolution in FOL Mechanical proof procedure with a single rule Invented by J. A. Robinson in 1965 Three main steps: 1. Clause form: transform all clauses to uniform format 2. Unification: find a variable assignment so that two clauses can be resolved 3. Resolution: rule to obtain new conclusions

5 1. Clause Form We want a uniform representation for our logical propositions, so that we can use a simple uniform proof procedure. Clausal form expresses all logical propositions using literals (atomic sentences or negated atomic sentences) and clauses (sets of literals representing their disjunction). Ex: all the following in a single form: a => b ~a \/ b b \/ ~a ~(~a /\ ~b)

6 Any set of sentences in predicate calculus can be converted to clausal form. Eight-step procedure for conversion: 1. Replace implications 2. Distribute negations 3. Standardize variables 4. Replace existentials 5. Remove universals 6. Distribute disjunctions 7. Replace operators 8. Rename variables Clause Form Conversion

7 1. Replace implications Any sentence of the form:  becomes (   becomes  \/  at all levels 2. Distribute negations Repeatedly do the following: ~~  becomes   /\  becomes  \/   \/  becomes  /\     becomes       becomes   

8 3. Standardize variables Rename variables to ensure that each quantifier has its own unique variable. Ex: (  X p(X)) /\ (  X q(X)) becomes (  X p(X)) /\ (  Y q(Y)) 4. Replace existentials Consider: (  Y  X p(X,Y)) The identity of X depends on the value of Y. We can replace X by a function of Y: (  Y p(g(Y),Y)) where g is a Skolem function

9 Skolemization Replace each occurrence of an existentially quantified variable by a Skolem function. The function’s arguments are the universally quantified variables that are bound by universal quantifiers whose scope includes the scope of the existential quantifier being eliminated. The Skolem functions must be new, i.e., not already present in any other sentences. If the existential quantifier is not within a universal quantifier, use a Skolem function of no arguments, i.e., a constant.

10 Examples of Skolemization 1.  X p(X) becomes p(g) 2.  Y  X p(X,Y) becomes  Y p(g(Y),Y) 3.  X  Y p(X,Y) becomes  Y p(g,Y) 4.  Y  X (((q(X) /\ p(X,Y)) \/  Z  W(r(W))) becomes  Y (((q(f(Y)) /\ p(f(Y),Y)) \/  Z (r(g(Y,Z))) where f and g are Skolem functions

11 5. Remove universal quantifiers Throw away  ’s and assume all variables are universally quantified.  X  Y  Z q(h(Y)) /\ p(h(Y),X) /\ r(X,Z) becomes q(h(Y)) /\ p(h(Y),X) /\ r(X,Z) 6. Distribute disjunctions Write the formula in conjunctive normal form (the conjunction of sets of disjunctions) with:  \/  /\  becomes (  \/  /\  \/ 

12 7. Replace operators Replace the conjunction S 1 /\ S 2 /\ …. /\ S n with the set of clausesS 1, S 2, …, S n Convert each S i into a set of literals (that is, get rid of the disjunctions symbols “ \/”, and write them in clause notation. Ex: (p(X) \/ q(Y)) /\ (p(X) \/ ~r(X,Y)) becomes two clauses: {p(X), q(Y)} {p(X), ~r(X,Y)}

13 8. Rename variables Change variable names so that no variable symbol appears in more than one clause: Ex: {p(X), q(Y)} and {p(X), ~r(X,Y)} becomes {p(X), q(Y)} and {p(Z), ~r(Z,W)} we can do this because:  X  Y(p(X) \/ q(Y)) /\ (p(X) \/ ~r(X,Y)) is equivalent to  X  Y(p(X) \/ q(Y)) /\  Z  W(p(Z) \/ ~r(Z,W))

14 Example of entire conversion (1)  X (p(X) => ((  Y p(Y) => (p(f(X,Y))) /\ (~  Y (~q(X,Y) \/ p(Y)))) 1.  X (~p(X) \/ ((  Y ~p(Y) \/ (p(f(X,Y))) /\ (~  Y (~q(X,Y) \/ p(Y)))) 2.  X (~p(X) \/ ((  Y ~p(Y) \/ (p(f(X,Y)) /\ (  Y (q(X,Y) /\ ~p(Y)))) 3.  X (~p(X) \/ ((  Y ~p(Y) \/ (p(f(X,Y))) /\ (  W (q(X,W) /\ ~p(W))))

15 Example of entire conversion (2) 4.  X (~p(X) \/ ((  Y ~p(Y) \/ (p(f(X,Y))) /\ (q(X,g(X)) /\ ~p(g(X))))) 5. ~p(X) \/ ((~p(Y) \/ (p(f(X,Y))) /\ (q(X,g(X)) /\ ~p(g(X))))) 6. (~p(X) \/ ~p(Y)) \/ (p(f(X,Y))) /\ (~p(X) \/ (q(X,g(X))) /\ (~p(X) \/~p(g(X)))))

16 Example of entire conversion (3) 7. {~p(X), ~p(Y)), p(f(X,Y))}, {~p(X), q(X,g(X)))}, {~p(X), ~p(g(X))} 8. {~p(X1), ~p(Y1)), p(f(X1,Y1))}, {~p(X2), q(X2,g(X2)))}, {~p(X3), ~p(g(X3))}

17 Unification -- Substitution Unification is the process of determining whether two expressions can be made identical by appropriate substitutions for their variables. The substitution applies to variables of both expressions and makes them syntactically equivalent. The result is a substitution instance Example of a unifying substitution S 1 : p(X,f(b),Z) S 2 : p(a,Y,W) U = {X/a Y/f(b), Z/W} S 1 [U] = S 2 [U] = p(a,f(b),W) substitution instance

18 Properties of the substitution 1. Each variable is associated with at most one expression; 2. No variable with an associated expression occurs within any associated expression. That is, no “left side” appears on a “right side”: U = {X/a, Y/f(X), Z/Y} is not a legal substitution

19 A set of expressions {S 1,S 2,….S n } is unifiable if there is a substitution U that makes them identical: S 1 [U] = S 2 [U] =… = S n [U] Ex: U = {X/a, Y/b, Z/c} unifies the expressions p(a,Y,Z) and p(X,b,Z) and p(a,Y,c) Unification of sentences

20 Two expressions can have more that one unifier that makes them equivalent: S 1 = p(X,Y,Y) and S 2 = p(a,Z,Z) are unified by U 1 ={X/a,Y/Z} and U 2 ={X/a,Y/b, Z/b} S 1 [ U 1 ] = S 2 [ U 1 ] = p(a,Z,Z) S 1 [ U 2 ] = S 2 [ U 2 ] = p(a,b,b) Unifiers are not unique!

21 Partial order between unifiers Some unifiers are more general than others: U 1 is more general than U 2 if there exists a unifier U 3 such that U 1 U 3 = U 2 Example: U 1 = {X/f(Y),Z/W,R/c} is more general than U 2 = {X/f(a),Z/b, R/c} since U 3 = {Y/a,W/b} U 1 U 3 ={X/f(Y),Z/W,R/c}{Y/a,W/b} = U 2 Unifiers form a partial order

22 Most general unifier (MGU) For each set of sentences, there exists a most general unifier (mgu) that is unique up to variable renaming Ex: for S 1 = p(X,Y,f(Z)) and S 2 = p(a,W,R) U={X/a,Y/W,R/f(Z)} is the mgu For the resolution procedure, we want to find the most general unifier of literals: a constant, variable, or a functional object of length n.

23 Basic functions for MGU procedure Constant(exp) returns true if exp is a constant Ex: a, f(g(a,b,c)) are constants Variable(exp) returns true if exp is a simple variable Length(exp) returns the number of items in a function Ex: f(a,g(Y)) is an object of length 2. MguVar (Var, exp) returns –false if Var is included in exp –{Var/exp} otherwise

24 Recursive procedure for MGU function Mgu(exp1,exp2) returns unifier if exp1 = exp2 return {} if Variable(exp1) then return MguVar(exp1,exp2) if Variable(exp2) then return MguVar(exp2,exp1) if Constant(exp1) or Constant(exp2) return false if not(Length(exp1) = Length(exp2)) return false unifier := {} for i := 1 to Length(exp1) do s := Mgu(Part(exp1,i),Part(exp2,i)) if s is false return false unifier := Compose(unifier,s) exp1 := Substitute(exp1,unifier) exp2 := Substitute(exp2,unifier) return unifier; end

25 Find the mgu unifier for p(f(X,g(a,Y)), g(a,Y)) and p(f(X,Z),Z). 1. Mgu is called on p and p, returns {} 2. Mgu is called recursively on f(X, g(a,Y)) and f(X,Z) 3. Mgu is called on f and f, returns {} 4. Mgu is called on X and X, returns {} 5. Mgu is called on g(a,Y) and Z; since Z is a variable, it returns (via Mguvar) {Z/g(a,Y)} after checking that Z is not in g(a,Y). Example of MGU trace (1)

26 Example of MGU trace (2) 6. {Z/g(a,Y)} is composed with the previous (empty) substitution 7. The entire substitution is applied to both expressions, yieldingf(X, g(a,Y)) 8. Since i = 3, Mgu returns {Z/g(a,Y)}, which is then applied to the top level expressions. All other checks show equality. The result is p(f(X,g(a,Y)),g(a,Y))

27 Given a clause containing the literal  and another clause the literal ~ , we can infer the clause consisting of all the literals of both clauses without  and ~ . Ex:1. {p, q}  2. {~q, r}  3. {p, r}1,2 3. Resolution

28 Why does this work? Consider the two clauses {winter, summer} {~winter, cold} At any point, winter is either true or false. If it is true, then cold must be true to guarantee the truth of clause 2. If winter is false, then summer must be true to guarantee the truth of clause 1. So regardless of winter's truth value, either cold or summer must be true, i.e., we can conclude: {cold, summer}

29 Other Resolution Examples 1. {p, q}  2. {~p, q}  3. {q}1,2 1. {~p, q}  2. {p}  3. {q} 1,2 1. {p}  2. {~p}  3. {}1,2 merge the q's much like Modus Ponens We can derive the empty clause, showing that the KB has a contradiction

30 For clauses containing variables, we can resolve  in one clause with ~  in another clause, as long as  and  have a mgu U. The resulting clause is the union of the original 2 clauses, with  and ~  removed, and with U applied to the remaining literals.   and   [U],  [U]} where  [U] =  [U] Resolution Rule

31 Ex1: 1. {p(X), q(X,Y)}  2. {~p(a), r(b,Z)}  3. {q(a,Y), r(b,Z)}1,2 Ex2: Two clauses may resolve in more than one way since  and ~  may be chosen in different ways: 1. {p(X,X), q(X), r(X)}  2. {~p(a,Z), ~q(b)}  3. {q(a), r(a),~q(b)}1,2 4. {p(b,b), r(b), ~p(a,Z)}1,2 Examples of Resolutions

32 A resolution deduction of a clause  from a data base  is a sequence of clauses in which 1.  is an element of the sequence; 2. Each element is either a member of  or the result of applying the resolution principle to clauses earlier in the sequence. Resolution Deduction

33 Resolution procedure with nondeterministic choices function Resolution(KB) returns answer while not(Empty_Clause(KB)) do c1 := Choose_Clause(KB) c2 := Choose_Clause(KB) res := Choose_Resolvents(c1,c2) KB := KB U {res} end return true end

34 Ex: Resolution deduction of the empty clause 1. { p }  2. {~p, q }  3. { ~q, r }   ~r }  5. {q}1, 2 6. { ~q}3, 4 7. { }5, 6

35 {p}{~p,q}{~q,r}{~r} {q} {~p,r} {~q} {r} { }{~p} A “resolution trace” is a linear form of the graph All possible resolutions (to 3 levels)

36 As for Propositional Logic, we will use refutation resolution as the single rule for proving sentences about a KB We will use it to prove the unsatisfiability of a set of clauses, i.e., they contain a contradiction if we can derive the empty clause. Resolution refutation: to prove c, prove that KB U {~c} |= 0 like a “proof by contradiction” Rule is refutation-complete. Resolution Refutation

37 Answering questions with Refutation-Resolution True/False questions: we want to know if a conclusion follows from KB: Ex:father(art, jon) father(bob, kim) father(X, Y) => parent(X, Y) Is art the parent of jon? Queries: fill-in-the blank: we want to know also what instantiation makes it true: Who is the parent of jon?

38 True/False by Refutation Resolution 1. {father(art, jon)}  2. {father(bob, kim)}  3. {~father(X,Y), parent(X,Y)}  4. {~parent(art,jon)} 5. {parent(art,jon)}1, 3 6. {parent(bob, kim)}2, 3 7. {~father(art, jon)}3, 4 8. { }4, 5 9. { }1, 7 negated goal clause

39 To obtain one instantiation that makes the conclusion true (if any), form a disjunction of the negation of the goal c and its “answer literal”: a term of form ans(X 1,X 2,…X n ) where the variables X 1,X 2,…X n are the free variables in c. Ex: Add to the previous KB the clause {~parent(X, jon), ans(X)} Resolution halts when it derives a clause containing only the ans literal Fill-in-the-blank (Green’s method)

40 Example of fill-the-blank derivation 1. {father(art, jon)}  2. {father(bob, kim)}  3. {~father(X,Y), parent(X,Y)}  4. {~parent(Z,jon), ans(Z)}c 5. {parent(art, jon)}1, 3 6. {parent(bob, kim)}2, 3 7. {~father(w,jon), ans(W)}3, 4 8. {ans(art) }4, 5 9. {ans(art) }1, 7

41 The answer may not be unique: several different answers may result from the proof, depending on the clauses that were chosen for resolution. We may also get an answer of the form {ans(a), ans(b) } where one of the answers is right, but the clause doesn’t tell us which one. 1. {father(art,jon), father(bob,jon)} KB 2. {~ father(X,jon),ans(X)} c 3. {father(bob,jon),ans(art)} 1, 2 4. {ans(art),ans(bob)} 2, 3 Properties of Fill-in-the-blank

42 Used to make the resolution procedure more efficient for predicates Attach a procedure to a predicate symbol or function symbol, then evaluate the expression. Ex: a procedure Greaterp is attached to the symbol >. When “2” is associated with the number 2 and “5” is associated with 5, we can evaluate >(5,2) by running the program Greaterp(5,2). {p(X), q(X), >(5, 2)} can be eliminated (its false) {p(X), q(X), <(5, 2)} becomes {p(X), q(X) } Procedural Attachement