An ethic is rational if it can justify itself rationally—that is to say, if there is a “why” and a “wherefore” in it amenable to reflection, and underivative. An ethic, on the other hand, is irrational if reason and reflection are irrelevant to it, or if, being relevant, they are fundamentally subordinate, and are only the lackeys of a governing consideration which is either irrational or non-rational. The intention of this lecture is to explore the possibilities of rationalism in ethics, supposing that the meaning of rationalism is, broadly speaking, what has just been stated.