Austin

Natural law theory exaggerates the relation of law and morality. Positive law is a reaction against particularly that aspect of Natural law theory. It insists on a distinction between human law, which they call positive law and moral and scientific laws. Human laws are posits of human society while scientific laws are independent of what we take them to be. (You will note, sometimes, that writers refer to what I call 'mores' as 'positive morality' in contrast to 'rational morality'.)
The Classical version of positive law theory is John Austin's (1797-1859)"command theory." His model was that of a definition and his goal was to give a definition of law that removed all evaluative language. We see some continuity with Aquinas' natural law. While he rejected the blurring of law and morality, he did give a similar "unified" definition of law: "A rule laid down for the guidance of an intelligent being by an intelligent being having power over him." God and men both make laws so his distinction is between the laws of God (reason) and those of historical human societies made by political "superiors." He insisted on distinguishing the theory of (concept of) law from the "science of legislation" which had to do with the criticism (evaluation) of the law (c.f. Dworkin's theory of legislative justice).
Austin is a prime example of a positivist in legal theory, but his was only one version which we call "command theory:" Law, Austin reasons, has the status of command. Austin then defines 'command" as any signification of a desire by the sovereign.   He then defines the sovereign as "the determinate rational being or body that the other rational beings are in the habit of obeying." Each of these further definitions is an attempt to substitute a descriptive analysis of some prescriptive concept. The notion of a 'command', for example, includes a normative element of authority and imperative (as distinct from a presumptive request). Similarly 'sovereign' has a normative element...