Philosophers since ancient times have pondered how we can know whether moral claims are true or false. Aristotle (386–322 B.C.) called attention to this concern in his Nicomachean Ethics: “Now fine and just actions admit of much variety and fluctuation of opinion, so that they may be thought to exist only by convention, and not by nature.” The first half of the twentieth century witnessed widespread skepticism concerning the possibility of moral knowledge. Indeed, some argued that moral statements lacked cognitive content altogether, because they were not susceptible to empirical verification. The British philosopher A. J. Ayer, for example, contended in his seminal work, Language, Truth, and Logic (1936), that “sentences which simply express moral judgements do not say anything. They are pure expressions of feeling and as such do not come under the category of truth and falsehood. They are unverifiable for the same reason as a cry of pain or a word of command is unverifiable – because they do not express genuine propositions.”

The second half of the twentieth century brought a revival of interest among philosophers in moral and political questions. Part of that revival consisted in a vigorous debate over the veracity of the earlier skepticism over the possibility of moral knowledge. Whether or not ethics can be founded upon a rational basis continues to roil the philosophical community at the dawn of the new century, with no consensus likely on these enduring questions: Can morality be founded upon facts about human nature, social agreement, volition, subjective preference, a priori reasoning, intuition, or some other basis?