are thus not purely descriptive — caused some to regard him as an emotivist, a classification he denied: Proponents Influential statements of emotivism were made by C. K. Ogden and I. A. Richards in their 1923 book on language, The Meaning of Meaning, and by W. H. F. Barnes and A. Duncan-Jones in independent works on ethics in 1934. However, it is the later works of Ayer and especially Stevenson that are the most developed and discussed defenses of the theory. A. J. Ayer A. J. Ayer's version of emotivism is given in chapter six, "Critique of Ethics and Theology", of Language, Truth and Logic. In that chapter, Ayer divides "the ordinary system of ethics" into four classes: "Propositions that express definitions of ethical terms, or judgements about the legitimacy or possibility of certain definitions" "Propositions describing the phenomena of moral experience, and their causes" "Exhortations to moral virtue" "Actual ethical judgments" He focuses on propositions of the first class—moral judgments—saying that those of the second class belong to science, those of the third are mere commands, and those of the fourth (which are considered in normative ethics as opposed to meta-ethics) are too concrete for ethical philosophy. While class three statements were irrelevant to Ayer's brand of emotivism, they would later play a significant role in Stevenson's. Ayer argues that moral judgments cannot be translated into non-ethical, empirical terms and thus cannot be verified; in this he agrees with ethical intuitionists. But he differs from intuitionists by discarding appeals to intuition as "worthless" for determining moral truths, since the intuition of one person often contradicts that of another. Instead, Ayer concludes that ethical concepts are "mere pseudo-concepts": Ayer agrees with subjectivists in saying that ethical statements are necessarily related to individual attitudes, but he says they lack truth value because they cannot be properly understood as propositions about those attitudes; Ayer thinks ethical sentences are expressions, not assertions, of approval. While an assertion of approval may always be accompanied by an expression of approval, expressions can be made without making assertions; Ayer's example is boredom, which can be expressed through the stated assertion "I am bored" or through non-assertions including tone of voice, body language, and various other verbal statements. He sees ethical statements as expressions of the latter sort, so the phrase "Theft is wrong" is a non-propositional sentence that is an expression of disapproval but is not