Predicate Logic (PL) 1.Syntax: The language of PL Formula VS. Sentence Symbolize A-, E, I-, and O-sentences 2.Semantic: a sentence compare two sentences compare a set of sentences 3.Derivation
Syntax: The language of PL SSingular term: a, b, c, …v PPredicate term: A’, B’, B’’, C’’,D…X, Y, Z,.. VVariables: w, x, y, z QQuantifier: (universal quantifier) (existential quantifier) Main logical operator: &, ~, , , FFormula: A, B, C, Ax, Ba, Cxy, Cb, ( x)Gx
Formula VS. Sentence Formula atomicRabz, Hxy, Hat, Gxy… truth-functional ~(Rabz & Hxy), Rabz & Hxy… quantificational ( x) (Hay ( x) Gxy), ( x) Gxy… Sentence atomicRabz, Ga, Az, Le… truth-functional Rz & Ga, ~Ba, (Le Ca) Ba… quantificational ( x) (Gx & Sax), ( x) Gxy, … 一個句子不能含有自由變元 (free variable)
A recursive definition of formula of PL 1.Every atomic formula of PL is a formula of PL 2.If P is a formula of PL, so is ~ P 3.If P and Q are formula of PL. so are ( P & Q ), ( P Q ), ( P Q ), and ( P Q ) 4.If P is a formula of PL that contains at least one occurrence of x and no x-quantifier, then ( x) P and ( x) P are both formula of PL. 5.Nothing is a formula of PL unless it can be formed by repeated applications of clauses 1-4.
Symbolize (1) UD: all people Lxy: x likes y (1) Every likes someone. ( x) ( y) Lxy (2) Someone likes everyone. ( x) ( y)Lxy (3) Everyone is liked by someone. ( x) ( y) Lyx
Symbolize (2) UD: everything Lxy: x likes y Px: x is a person (1) Everyone likes someone. ( x) (Px ( y) (Py & Lxy) (2) Someone likes everyone. ( x) (Px & ( y) (Px Lxy) (3) Everyone is liked by someone. ( x) (Px ( y) (Px & Lyx)
A-, E, I-, and O-sentences A: All S are PE: No S are P I: Some S are P I: Some S are not P ( x) (Sx Px) =~( x) (Sx & ~Px) ( x) (Sx ~Px) =~( x) (Sx & Px) ( x) (Sx & Px) =~( x) (Sx ~Px) ( x) (Sx & ~Px) =~( x) (Sx Px) contradictory