ABSTRACT
Contraposition Basic notion of logic. In sentential logic, turning a sentence 'If p then q' around to 'If not-q then not-p' (called the contrapositive of the original sentence) is contraposition. This is valid in most logics of conditionals. In Aristotelian logic, contraposition refers to the valid immediate inference forms 'All A are B; therefore all non-Bare non-A' and 'Some A are not B; therefore some non-Bare not non-A' and their converses. See Immediate inference.
