Moral realism entails that there are metaphysically necessary moral principles of the form ‘all actions of nonmoral kind Z are morally good’; being discoverable a priori, these must be (in a wide sense) logically necessary. This article seeks to justify this apparently puzzling consequence. A sentence expresses a logically necessary proposition iff its negation entails a contradiction. The method of reflective equilibrium assumes that the simplest account of the apparently correct use of sentences of some type in paradigm examples is probably logically necessary. An account is simple insofar as it uses few predicates designating properties easily recognizable in many different kinds of paradigm examples. I illustrate how reflective equilibrium uses these criteria to discover logically necessary nonmoral propositions such as ‘if S remembers doing X, S believes that he did X’ and ‘if it is scarlet, it is red’. I then illustrate how exactly the same procedures can lead us from paradigm examples of apparently true sentences asserting that actions of some kind are good to discovering moral principles. I advance the contingent hypothesis that most contemporary humans, although they initially disagree about moral issues, have derived from different paradigm examples the same concept of moral goodness, which will ensure that the use of reflective equilibrium will lead to eventual agreement about moral issues. That strongly suggests that the moral principles eventually reached by reflective equilibrium are logically necessary ones.