  Legal theorists who present or understand their theories as “positivist”, or as instances of “legal positivism”, take their theories to be opposed to, or at least clearly distinct from, natural law theory. Natural law theorists, on the other hand, did not conceive their theories in opposition to, or even as distinct from, legal positivism (contra Soper 1992 at 2395). The term “positive law” was put into wide philosophical circulation first by Aquinas, and natural law theories of his kind share, or at least make no effort to deny, many or virtually all “positivist” theses—except of course the bare thesis that natural law theories are mistaken, or the thesis that a norm is the content of an act of will. Natural law theory accepts that law can be considered and spoken of both as a sheer social fact of power and practice, and as a set of reasons for action that can be and often are sound as reasons and therefore normative for reasonable people addressed by them. This dual character of positive law is presupposed by the well-known slogan “Unjust laws are not laws.” Properly understood, that slogan indicates why—unless based upon some skeptical denial that there are any sound reasons for action (a denial which can be set aside because defending it is self-refuting)—positivist opposition to natural law theories is pointless, that is redundant: what positivists characteristically see as realities to be affirmed are already affirmed by natural law theory, and what they characteristically see as illusions to be dispelled are no part of natural law theory. But because legal theories conceived of by their authors as positivist are, by and large, dominant in the milieux of those likely to be reading this entry, it seems appropriate to refer to those theories along the way, in the hope of overcoming misunderstandings that (while stimulating certain clarifications and improvements of natural law theorizing) have generated some needless debate.