Full Idea
The Law of Transposition says that (P→Q) → (¬Q→¬P).
Gist of Idea
The Law of Transposition says (P→Q) → (¬Q→¬P)
Source
GE Hughes/M Cresswell (An Introduction to Modal Logic [1968], Ch.1)
Book Reference
Hughes,G./Cresswell,M.: 'An Introduction to Modal Logic' [Methuen 1972], p.15
A Reaction
That is, if the consequent (Q) of a conditional is false, then the antecedent (P) must have been false.