Among the most-influential philosophers of law from the early modern period was Thomas Hobbes (1588–1679), whose theory of law was a novel amalgam of themes from both the natural-law and command-theory traditions. He also offered some of the earliest criticisms of common-law theory, which would be developed significantly by theorists in the 18th century. For Hobbes, law was the primary instrument of a sovereign by which to serve the ends of government, which were principally peace and the personal security of all its citizens. Writing during and after the English Civil Wars (1642–51), he developed the idea that government which ruled effectively by law is the only bulwark against anarchy or, as he famously put it, “a war of all against all.” Hobbes’s philosophy of law is in part an account of what law must be like in order to serve that function. Many scholars credit Hobbes as the founder of legal positivism, the dominant philosophical theory of law since the 17th century. The core ideas of legal positivism are that law is essentially a matter of social fact and that it bears at most a contingent connection with moral norms: many actions that are legally proscribed (or prescribed) can nonetheless be moral (or immoral). Insofar as this was Hobbes’s view, it was because he was an adherent of the command theory of law already discussed. In his magnum opus, Leviathan (1651), he wrote that “law in general, is not counsel, but command” and that civil (i.e., positive) laws are “those rules which the common-wealth hath commanded…by word, writing, or other sufficient sign of the will” that certain actions are to be done or not done. Since laws are “signs of the will” of the sovereign, Hobbes placed particular emphasis on the requirement that those “signs” are sufficiently public and intelligible to ordinary citizens.

Hobbes’s fundamental criticism of common-law theory was that the “immemorial customs” of the community, claimed to be the foundations of law, are not always easily discernible; they may in fact be deeply controversial, and so the common law may by nature fail to offer authoritative and final views of what its putative subjects ought to do. Hobbes rejected Coke’s idea that coming to know the law required an exercise of “artificial reason” and “long study and experience,” arguing that if lawyers and judges were necessary intermediaries between sovereign and subject, then the law would again fail to guide the conduct of those to whom it applied. He quipped that ordinary persons could dispense with the counsel of lawyers and master the contents of a legal system after about two months’ study.

Although there are undeniable positivist elements in Hobbes’s theory, in positing an important connection between natural and civil law (i.e., between morality and positive law), he was also inspired by the natural-law tradition. He claimed that natural law and civil law “contain each other and are of equal extent.” What Hobbes meant by that claim has been a topic of scholarly debate ever since; suffice it to say that he thought that there were modest but real moral limits on what the sovereign could legitimately demand of its subjects. For example, a putative law that required people to act in ways that led to their own death would fail to be valid positive law because it would violate the natural law of self-preservation, which Hobbes thought was at the foundation of the purpose of government. Hobbes thus attempted a synthesis of the natural-law and command traditions, though some scholars think he was far from successful.