Full Idea
All pairs of contraries entail a pair of contradictories, since one member of such a pair always entails the negation of the other. P&Q and not-P are contraries, but the first entails P, which is contradictory of not-P.
Gist of Idea
Contrary pairs entail contradictions; one member entails negation of the other
Source
Peter Lipton (Inference to the Best Explanation (2nd) [2004], 09 'Is the best')
Book Reference
Lipton,Peter: 'Inference to the Best Explanation (2nd ed)' [Routledge 2004], p.156