A rule of inference used in prepositional logic. It is often abbreviated to 'Trans' for short.
(P→Q) = (¬Q →¬P)
If P, then Q is the same as If 'not Q' than 'not P'.
And, of course, vice versa.
Think about it. Think hard. And when that doesn't work, construct a Truth Table.
Oi. Okay, look. If P happens, then Q happens. That's the (P→Q) bit. So, if Q didn't happen, you know for a fact that P didn't happen, right? Cause if it did, you would have Q. So, not Q, then not P.
Back up to Rules of Inference
Review your Logic symbols