For more than 100 years, metaethicists have overlooked the best version of G. E. Moore’s Open Question argument. This despite the fact that it appears on the same page of Principia Ethica as his other, weaker versions of the argument. This better Open Question Argument does not rely on introspection of the meanings of ethical terms, and so does not fall to the standard criticisms of Moore. In this paper, I present this ‘new’ Open Question Argument and show that Moore (...) has done to naturalistic ethics something like what Plato’s Euthyphro does to supernaturalistic ethics. (shrink)