Propositional Logic Not Enough • If we have: ○ “All men are mortal.” ○ “Socrates is a man.” • Does it follow that “Socrates is mortal?” • Can’t be represented in propositional logic. • Need a language that talks about objects, their properties, and their relations. • Later we’ll see how to draw inferences. Introducing Predicate Logic • Predicate logic uses the following new features: ○ Variables: x, y, z ○ Predicates: P(x), M(x) ○ Quantifiers: exists and for all • Propositional functions are a generalization of propositions. ○ They contain variables and a predicate, e.g., P(x) ○ Variables can be replaced by elements from their domain. Propositional Functions • Propositional functions become propositions (and have truth values) when their variables are each replaced by a value from the domain (or bound by a quantifier). • The statement P(x) is said to be the value of the propositional function P at x. • For example, let P(x) denote “x0” and the domain be the integers. Then: ○ P(−3) is false. ○ P(0) is false. ○ P(3) is true. • Often the domain is denoted by U. So in this example U is the integers. Examples of Propositional Functions • Let “x+y=z” be denoted by R(x, y, z) and U be the integers. • Find these truth values: ○ R(2,−1,5) =F ○ R(3,4,7)=T ○ R(x, 3, z)⇒ Not a Proposition • Now let “x is the least number” be denoted by Q(x), with U={0,1,2,3,5}. • Find these truth values: ○ Q(0)=T ○ Q(5)=F ○ Q(6)⇒ undefined • What is Q(0) is U is the integers? Q(0)=F Compound Expressions • Connectives from propositional logic carry over to predicate logic. • If P(x) denotes “x0,” find these truth values: ○ P(3)∨ P(−1)=T∨F=T ○ P(3)∨ P(−1)=T∧F=F ○ P(3)→P(−1)=T→F=F ○ P(3)→¬P(−1)=T→T=T • Expressions with variables are not propositions and therefore do not have truth values. For example, ○ P(3)∧P(y) ○ P(x)→P(y) • When used with quantifiers (to be introduced next), these expressions (propositional functions) become propositions. Quantifiers • We need quantifiers to express the meaning of English words including all and some: ○ “All men are Mortal.” ○ “Some cats do not have fur.” • The two most important quantifiers are: ○ Universal Quantifier, “For all,” symbol: ∀ ○ Existential Quantifier, “There exists,” symbol: ∃ • We write as in ∀x P(x) and ∃x P(x). ○ ∀x P(x) asserts P(x) is true for every x in the domain. ○ ∃x P(x) asserts P(x) is true for some x in the domain. • The quantifiers are said to bind the variable x in these expressions. Universal Quantifier • ∀x P(x) is read as “For all x, P(x)” or “For every x, P(x)” • If P(x) denotes “x0” and U is the integers, then ∀x P(x) is false. • If P(x) denotes “x0” and U is the positive integers, then ∀x P(x) is true. • If P(x) denotes “x is even” and U is the integers, then ∀x P(x) is false. Existential Quantifier • ∃x P(x) is read as “For some x, P(x)”, or as “There is an x such that P(x),” or “For at least one x, P(x).” • If P(x) denotes “x0” and U is the integers, then ∃x P(x) is true. It is also true if U is the positive integers. • If P(x) denotes “x0” and U is the positive integers, then ∃x P(x) is false. • If P(x) denotes “x is even” and U is the integers, then ∃x P(x) is true. Thinking about Quantifiers • When the domain of discourse is finite, we can think of quantification as looping through the elements of the domain. • To evaluate ∀x P(x) loop through all x in the domain. • If at every step P(x) is true, then ∀x P(x) is true. • If at a step P(x) is false, then ∀x P(x) is false and the loop terminates. • To evaluate ∃x P(x) loop through all x in the domain. • If at some step, P(x) is true, then ∃x P(x) is true and the loop terminates. • If the loop ends without finding an x for which P(x) is true, then ∃x P(x) is false. • Even if the domains are infinite, we can still think of the quantifiers this fashion, but the loops will not terminate in some cases. Thinking about Quantifiers as Conjunctions and Disjunctions • If the domain is finite, a universally quantified proposition is equivalent to a conjunction of propositions without quantifiers and an existentially quantified proposition is equivalent to a disjunction of propositions without quantifiers. • If U consists of the integers 1,2, and 3: ○ ∀x P(x)≡P(1)∧P(2)∧P(3) ○ ∃x P(x)≡P(1)∨P(2)∨P(3) • Even if the domains are infinite, you can still think of the quantifiers in this fashion, but the equivalent expressions without quantifiers will be infinitely long. Precedence of Quantifiers • The quantifiers ∀ and ∃ have higher precedence than all the logical operators. • For example, ∀x P(x)∨Q(x) means (∀x P(x))∨Q(x) • ∀x (P(x)∨Q(x)) means something different. Translating from English to Logic • Every student in this class has taken a course in Java. ○ Solution 1 § If U=every student in the class § Let J(x)≔x has taken a course in Java § ∀x J(x) ○ Solution 2 § If U= every student § Let C(x)≔x is a student in the class § Let J(x)≔x has taken a course in Java § Let ∀x (C(x)→J(x)) • Some but not all students in this class has taken a course in Java. ○ Let C(x)≔x is a student in the class ○ Let J(x)≔x has taken a course in Java ○ Some but not all § ∃x J(x)∧¬∀x J(x) § ≡∃x J(x)∧ ∃x ¬J(x) ○ Solution § ∃x(C(x)∧J(x))∧¬∀x(C(x)→J(x)) § ≡∃x(C(x)∧J(x))∧¬∀x (C(x)→J(x)) § ≡∃x(C(x)∧J(x))∧∀x ¬(C(x)→J(x)) § ≡∃x(C(x)∧J(x))∧ ∃x (C(x)∧¬J(x)) Equivalences in Predicate Logic • Statements involving predicates and quantifiers are logically equivalent if and only if they have the same truth value ○ for every predicate substituted into these statements and ○ for every domain of discourse used for the variables in the expressions. • The notation S≡T indicates that S and T are logically equivalent. • Example: ∀x ¬¬S(x) ≡ ∀x S(x) Negating Quantified Expressions • Consider ∀x J(x) ○ “Every student in your class has taken a course in Java.” ○ Here J(x) is “x has taken a course in Java” and ○ the domain is students in your class. ○ Negating the original statement gives “It is not the case that every student in your class has taken Java.” ○ This implies that “There is a student in your class who has not taken Java.” ○ Symbolically ¬∀x J(x) and ∃x ¬J(x) are equivalent • Consider ∃x J(x) ○ “There is a student in this class who has taken a course in Java.” ○ Where J(x) is “x has taken a course in Java.” ○ Negating the original statement gives “It is not the case that there is a student in this class who has taken Java.” ○ This implies that “Every student in this class has not taken Java” ○ Symbolically ¬∃x J(x) and ∀x ¬J(x) are equivalent Equivalent Statements • ∀x(P(x)∧Q(x))≡∀x P(x)∧∀x Q(x) • ∀x(P(x)∨Q(x))≠∀x P(x)∨∀x Q(x) ○ Let U=N={0,1,2,3…} ○ Let P(x)≔x is even ○ Let Q(x)≔x is odd ○ ∀x(P(x)∨Q(x)): every natural number is even or odd ○ ∀x P(x)∨∀x Q(x): every natural number is even or every natural number is odd • ∀x P(x)≡∀z P(z) Lewis Carroll Example • The first two are called premises and the third is called the conclusion. 1. “All lions are fierce.” 2. “Some lions do not drink coffee.” 3. “Some fierce creatures do not drink coffee.” • Define ○ U≔ all creatures ○ L(x)≔x is a lion ○ F(x)≔x is fierce ○ C(x)≔x drinks coffee • Translation ○ ∀x (L(x)→F(x)) ○ ∃x (L(x)∧¬C(x)) ○ ∃x(F(x)∧¬C(x)) Some Predicate Calculus Definitions • An assertion involving predicates and quantifiers is valid if it is true ○ for all domains ○ every propositional function substituted for the predicates in the assertion. • Example: ∀x ¬S(x)⟷¬∃x S(x) • An assertion involving predicates is satisfiable if it is true ○ for some domains ○ some propositional functions that can be substituted for the predicates in the assertion. • Otherwise it is unsatisfiable. • Example: ∀x (F(x)⟷T(x)) not valid but satisfiable • Example: ∀x(F(x)∧¬F(x)) unsatisfiable
