 That a concept always corresponds to something possessing a defining property is a very old idea. This assumption undergirds Plato’s Euthyphro and other dialogues in which Socrates pushes his interlocutors to make that hidden, defining property explicit, and this pursuit has provided a model for much not only of philosophy, but of the theorizing in all fields. The traditional assumption is that every entity has some essence that makes it the thing it is, and every instance that is accurately described by a concept of that entity will have that essence. The recent argument that there is an alternative structure—that a concept need not have necessary and sufficient criteria for its application—has been called a “conceptual revolution” (Needham 1975: 351), “one of the greatest and most valuable discoveries that has been made of late years in the republic of letters” (Bambrough 1960–1: 207).