 Given the standard theory, necessary and sufficient conditions are converses of each other: B’s being a necessary condition of A is equivalent to A’s being a sufficient condition of B (and vice versa). So it seems that any truth-functional conditional sentence states both a sufficient and a necessary condition as well. Suppose that if Nellie is an elephant, then she has a trunk. Being an elephant is a sufficient condition of her having a trunk; having a trunk in turn is a necessary condition of Nellie’s being an elephant. Indeed, the claim about the necessary condition appears to be another way of putting the claim about the sufficient condition, just as the contrapositive of a formula is logically equivalent to the original formula.