The claim that “‘is’ does not entail ‘ought’” is so closely associated with Hume that it has been called “Hume's Law.” The interpretation of the passage in Hume's Treatise of Human Nature that is the locus classicus of the claim is controversial. But the passage is preceded by three main bodies of argument, and, on the working assumption that the passage in question is closely connected to the argumentation that leads up to it, I will here examine the third of these, running from T 463:7 to 469:18.

While interpretations have differed from one another, they have agreed in attributing to Hume uncharacteristically weak arguments. I propose to show that Hume's arguments are both stronger and more interesting than has been allowed. But – I will argue – they exploit and consequently depend upon a semantic theory that contemporary philosophers are no longer able to accept.

Hume must be assigned a good deal of the responsibility for making “‘is’ does not entail ‘ought’” part of philosophers' (and not just philosophers') commonsense. If I am right both about Hume's influence and about the presuppositions of his arguments, then the interest of these conclusions is not merely historical. Today Hume's Law is a philosophical near-truism, and the burden of proof is taken to rest squarely on the shoulders of its opponents. But if Hume's Law is inherited from Hume, and was originally accepted on the basis of arguments that we can no longer find acceptable, this may require a reassessment of just where the burden of proof may be presumed to lie.