The naturalists claimed that morals were part of the natural world and could be discovered using reason. The Logical Positivists were scientific in their methods for verifying knowledge, and here Hume's influence can be seen in excluding the possibility of moral facts. Hume believed that sentiment was the source of right and wrong, nothing more. Moral judgements are made because of feelings and not reason, and it is not possible to move from a factual statement to a moral one.
Emotivism is a moral theory based on people's emotive responses to things and is a reductionism theory; it seeks to reduce moral statements to responses such as expressions of preference, attitude or feeling. It is for this reason that the theory is sometimes referred to as the "Boo/Hurrah" theory. Stevenson maintained that moral arguments are merely disagreements in attitude, but that the moral statements given are not merely expressions of emotion. They are the result of attitudes based on fundamental beliefs. Moral terms are persuasive and substantiate beliefs. In such a case the Logical Positivist view of verification is inadequate.
Does this constitute a moral theory? The main case that has been made against emotivism is that it is too reductionist; it reduces moral debate to "so much hot air" and nothing more. How are we able to be sure of anything? Based on emotivism we would be unable to distinguish between lying and truth-telling, murder or saving a life. Ayer rejected traditional normative explanations of ethics, including both teleological and deontological theories. Hare thought that the concept of universalisability - a Kantian ethic - should be introduced as an aid to resolving moral conflict. Stevenson gave more meaning to moral disagreements than Ayer but concluded that moral statements were always subjective - there was no universal truth or right or wrong. For MacIntyre this made emotivism a misconceived theory of ethics; he found social emotivism - in which moral judgements are seen as mere expressions of personal preference nothing more - disturbing.