Date: Jul 28, 2013 12:16 AM
Author: 何少仪
Subject: universal quantification vs existential quantifier

i delete former post,  beacuse it's format.


if set S(X) is 'x is the class's student', C(x) is 'x has learned math'.

so we cannot use ?x(S(x)->C(x)) (domain: x is all people) shows some students of class have learned math. because S(x) is false (x is not the class's student), S(x)->C(x) is true.

but why can i use ?x(S(x)->C(x)) (domain: x is all people) shows all the class's student have learned math,
beacuse S(x) is false (x is not the class's student), S(x)->C(x) is also true?