I'm doing the body of exercises in Priest's Introduction to Non-classical logic, and got weirdly stuck on 2.12 (o). (I'm on Problems 2.12, if anyone has a link to the answers that would also be much appreciated; can't find them online.) The exercise is to "show" that it is not the case that it is not a logical truth that necessarily p entails p and the method is to construct a tableau.
I start by negating it. 1) Not (necessarily p entails p). 2) necessarily p, not p (from 1).
Then I can't get any further with tableaux methods at least.