This paper proves a precisification of Hume’s Law—the thesis that one cannot get an ought from an is—as an instance of a more general theorem which establishes several other philosophically interesting, though less controversial, barriers to logical consequence.

The classical view of the relationship between necessity and apriority, defended by Leibniz and Kant, is that all necessary truths are known a priori. The classical view is now almost universally rejected, ever since Saul Kripke and Hilary Putnam discovered that there are necessary truths that are known only a posteriori. However, in recent years a new debate has emerged over the epistemology of these necessary a posteriori truths. According to one view – call it the neo-classical view – knowledge (...) of a necessary truth always depends on at least one item of a priori knowledge. According to the rival view – call it the neoempiricist view – our knowledge of necessity is sometimes broadly empirical. In this paper I present and defend an argument against the neo-empiricist view. I argue that knowledge of the necessity of a necessary truth could not be broadly empirical. (shrink)