Gibbard is best known in philosophy for his contributions to ethical theory. He is the author of three books in this area. "Wise Choices, Apt Feelings: A Theory of Normative Judgment" (1990) develops a general theory of moral judgment and judgments of rationality. Gibbard argues that when we endorse someone's action, belief, or feeling as "rational" or warranted we are expressing acceptance of a system of norms that permits it. More narrowly, morality is about norms relating to the aptness of moral feelings (such as guilt and resentment).