Entailment, as used by philosophers, is a term of art that, unlike logical consequence, lacks a precise definition that is consistently adhered to by those who employ it. Throughout much of the twentieth century, especially its early and middle years, many philosophers connected entailment with analyticity, requiring the material conditional ^┌A ⊃ B^┐ to be analytic when A entailed B. In later years, as conceptions of analyticity became less expansive, and philosophical uses of it more restricted, the presumption that entailment was to be understood in terms of analyticity waned. However, the relationship between entailment and necessity has remained robust. Standardly, when it is claimed that A entails B, B is taken to be a necessary consequence of A in the sense that it is impossible for A to be true without B's being true. Often, though not always, B is required to...