Emotivism... holds that the statements “you should be kind” and “murder is evil” are equivalent to saying “Yay, kindness!” and “Boo, murder!”. These statements express meaning non-cognitively, but are not propositions and do not have any truth value. http://www.philosophy-index.com/ethics/meta-ethics/non-cognitivism.php
A.J.Ayer’s influential 1936 book Language, Truth and Logic brought Logical Positivism to the English-speaking world. The central tenet of Logical Positivism is that the meaning of a statement is its mode of verification, which generally works well with physical facts (the meaning of ‘my cup is on the table’ is clear as I can look to check that this is indeed the case). Statements which cannot be verified are therefore considered meaningless. By some logical positivist thinking, then, moral statements are meaningless as they cannot be verified (where I should I look to verify whether it is wrong to do x?). It is thus a non-realist moral view, suggesting that there are no moral facts. Emotivism (also known as ‘hurrah/boo theory’) suggests that moral statements do not state propositions which describe the world in the same way which empirical statements do, but instead express emotional attitudes, e.g.:
"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"
A.J.Ayer’s, Language, Truth and Logic, 1936. pp103
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