Abstract. This essay explores the following position: Ultimate moral principles are a priori truths; hence, it is irrational to assign a non-zero credence to any proposition that is incompatible with these ultimate moral principles (since such propositions are a priori false); and this sort of irrationality, if it could have been avoided, is in a sense inexcusable. So – at least if moral relativism is false – in any disagreement about ultimate moral principles, at least one party to the disagreement is inexcusably irrational. This position may seem extreme, but it is argued that it is more plausible than it first appears.
Pre-reading is not required, but the paper is available upon request.