Moral rationalism, like many philosophical "isms," is an umbrella term for a variety of constituent claims. Not all moral rationalists endorse all of these claims, but the central ones that have been associated with moral rationalism are:

1) The metaphysical thesis: Basic moral requirements are constituted by the deliverances of sound practical reason.

2) The epistemological thesis: Humankind's basic moral requirements are knowable a priori.

3) The normative thesis: Moral requirements entail excellent reasons for action.

The metaphysical thesis is at the heart of practical reason approaches to ethics, such as those of Thomas Hobbes and Immanuel Kant, and, in modern times, David Gauthier, Christine Korsgaard, and Michael Smith. Though the views of these thinkers differ in many important respects, they agree that our basic moral duties are a function of sound practical reason. This means that something is a moral duty for a person...