I think the answer to the question I posed yesterday is this: history of philosophy, like any history, attempts to make what has happened understandable; and to make something understandable, usually we need to account for diverse phenomena, and unify them, in terms of single principle, or a small set of principles. A true theory can play the role of a single principle, when we interpret what came before as a progressive series of attempts to state the theory that we regard as true. (E.g. we see Copernicus and Kepler as antecedents to Newton; or Aristotle interpreted his predecessors in physical science as attempting to identify all four causes.) Yet it would seem that this is not the only way in which the phenomena of history may be unified. (Consider, e.g. the view that Hume is a sceptic who brings the epistemological turn initiated by Descartes to its logical conclusion. Two scholars, one who endorses Hume and the other who does not, might both think this: one man's modus ponens is another man's modus tollens.)
But now what about natural law theory: how do we define it?
Mark Murphy in his article, already mentioned, in SEP, defines it by giving a paradigmatic instance, viz. the theory of Thomas Aquinas:
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.This is a respectable way of arriving at a definition, no doubt, and perhaps Murphy is right in his choice of Aquinas as a paradigm; but surely his reasons for that choice are bizarre: "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." So he has made Prentice-Hall and Macmillan the determining authorities in this matter.
I had mentioned John Wild's book, Plato's Modern Enemies and the Theory of Natural Law. Wild arrives at his own definition by a reflection on the word 'nature' (phusis, natura). He distinguishes five things relevant to morality which, he thinks, are meant by this, and then he looks at 'the early Stoics', Marcus Aurelius, Thomas Aquinas, Richard Hooker, Hugo Grotius, and Thomas Paine, and argues that each of these thinkers, commonly accounted philosophers of 'natural law', organized their thought around these five distinguished senses of 'nature'. Wild's method thus combines etymological reflection (a venerable way of generating endoxa) with a consideration of paradigmatic cases.
Here's the definition that Striker gives at the beginning of her article. It seems a definition by stipulation. I may say something about the adequacy of this definition tomorrow:
The term "natural law" refers, it would seem, to the rules of morality conceived of as a kind of legal system, but one that has not been enacted by any human legislator. By contrast to human legal codes, the natural law is supposed to be valid independently of any formal procedures, and such that is cannot be changed. Besides, this law is supposed to provide the standards by which human legislation is to be judged--laws will be just or unjust depending on whether they do or do not conform to natural law. This is, at any rate, the concept of natural law that I'm going to talk about.But how else might one define 'natural law'?
In addition it might be defined pragmatically, I think. By this I mean: look to the contexts in which people have wanted to appeal to it; see what it is that they wished to accomplish by such appeal; and then ascribe to natural law just that content that would enable it to play that sort of role.
For instance, look to the appeal to natural law made by Martin Luther King, Jr., in his "Letter from a Birmingham Jail", or to the tribunal at Nuremberg, when it tried Nazis and Nazi collaborators for crimes, but not for any violations of German positive law; or to the appeal to natural law and natural rights made by American revolutionaries. How much content would we need to build into 'natural law' for it to play the public and political role it is meant to play in these contexts?