 Contraposition of entailment  Entailment is contrapositive, but not commutative. That is, it holds that \(e\) entails \(h\) \((e\vDash h)\) if and only if \(\neg h\) entails \(\neg e\) \((\neg h\vDash \neg e),\) while it does not hold that \(e\) entails \(h\) if and only if \(h\) entails \(e\) \((h\vDash e).\)