In these lectures Hare offers us a taxonomy of ethical theories, and then argues that the only plausible ethical theory is universal prescriptivism. Hare defines ethical theory as the study of the moral concepts, that is, of our use of moral words of their meaning in a broad sense, or of what we are doing when we ask moral questions. Broadly speaking, ethical theories come in two flavors, descriptivist and non-descriptivist. The former state that the meaning of a moral statement is determined by its truth conditions, that is, by the conditions under which it would correctly be said to be true. Non-descriptivist ethical theories deny this.
Descriptivisms differ according to the nature of the truth conditions they say determine meaning.
Objective naturalism cashes out the truth-value in terms that do not refer to the attitudes of individuals or groups. Subjective naturalism, on the other hand, cashes out the truth-value in terms that do refer to individual or group attitudes. Intuitionisms hold that the truth conditions of moral statements depend on whether or not acts, etc. Let us examine his arguments against objective and subjective naturalism and then his arguments against intuitionism. If two groups apply different moral predicates to one and the same act, for example, objective naturalism is committed to saying that each culture is correct in their sense of the moral term.