https://en.m.wikipedia.org/wiki/Contraposition https://en.m.wikipedia.org/wiki/Contraposition I imagine you’re remembering that P -> Q does not mean ~ P -> ~Q.

https://en.m.wikipedia.org/wiki/Contraposition https://en.m.wikipedia.org/wiki/Contraposition I imagine you’re remembering that P -> Q does not mean ~ P -> ~Q. That’s right, but you can absolutely get to ~Q -> ~P.