That G. E. Moore’s Principia Ethica has attained the status of a modern classic is amply attested by the number of references made to its central concepts and arguments. Moore’s central contention is that the adjective “good” refers to a simple, unique, and unanalyzable property. He claims that propositions containing value terms and ethical predicates are meaningful and can be found to be either true or false, even though the word “good” names an indefinable property knowable only by intuition or immediate insight. Moore also argues that the truth of propositions predicating intrinsic goodness—that is, that something is good on its own account, quite without reference to its value as a means—must likewise be seen immediately and without proof. The term “naturalistic fallacy” is proposed to name the error of mistaking some property other than goodness for goodness itself. Any definition of “good” would involve reference to something having distinguishable aspects or parts—hence, not simple; but since goodness is simple, any such definition would be false, an instance of the naturalistic fallacy.
The failure of previous systems of ethics, Moore alleges, is attributable to their imprecise formulations of the questions peculiar to ethics. His objective is to discover and lay down those basic principles according to which any scientific ethical investigation must proceed. Ethics should be concerned with two basic questions: “What kinds of things ought to exist for their own sakes?”—which presupposes knowledge of good—and “What kinds of actions ought we to perform?”