35.5.1 Converse and Contrapositive
Core ideaTurn it around, then flip and turn it: from "if p then q," the contrapositive "if ¬q then ¬p" has the same truth value.
Module goal. Link conditions, quantifiers, and negation into reasoning — using counterexamples, the contrapositive, and proof by contradiction — and pave the way for rigorous arguments about sequences and function limits.