By examining the theories of justice developed by Joseph Butler and David Hume, the author discloses the conceptual limits of their moral naturalism. Butler was unable to accommodate the possibility that justice is, at least to some extent, a social convention. Hume, who more presciently tried to spell out the conventional character of justice, was unable to carry through that project within the framework of his moral naturalism. These limits have gone unnoticed,largely because Butler and Hume have been misinterpreted, their relation misconstrued. Exegetes have persistently misunderstood the differences that divide them, have misconceived the notion of “public utility” in Hume's account of justice, have wrongly interpreted Butler as a forerunner of Immanuel Kant, and have altogether missed the degree to which Hume stands in the tradition of Thomas Aquinas.