 Even though we have already confined ‘natural law theory’ to its use as a term that marks off a certain class of ethical theories, we still have a confusing variety of meanings to contend with. Some writers use the term with such a broad meaning that any moral theory that is a version of moral realism — that is, any moral theory that holds that some positive moral claims are literally true (for this conception of moral realism, see Sayre-McCord 1988)— counts as a natural law view. Some use it so narrowly that no moral theory that is not grounded in a very specific form of Aristotelian teleology could count as a natural law view. It might be thought that there is nothing that can be done to begin a discussion of natural law theory in ethics other than to stipulate a meaning for ‘natural law theory’ and to proceed from there. But there is a better way of proceeding, one that takes as its starting point the central role that the moral theorizing of Thomas Aquinas plays in the natural law tradition. If any moral theory is a theory of natural law, it is Aquinas’s. (Every introductory ethics anthology that includes material on natural law theory includes material by or about Aquinas; every encyclopedia article on natural law thought refers to Aquinas.)  It would seem sensible, then, to take Aquinas’s natural law theory as the central case of a natural law position: of theories that exhibit all of the key features of Aquinas’s natural law view we can say that they are clearly natural law theories; of theories that exhibit few of them we can say that they are clearly not natural law theories; and of theories that exhibit many but not all of them we can say that they are in the neighborhood of the natural law view but nonetheless must be viewed as at most deviant cases of that position. There remain, no doubt, questions about how we determine what are to count as the key features of Aquinas’s position. But we may take as the key features those theses about natural law that structure his overall moral view and which provide the basis for other theses about the natural law that he affirms. 