Ayer takes subjectivism to be the thesis that actions are right or good if a person or group of persons approves of it. The subjectivist thinks, according to Ayer, that one can translate statement of value into statements of empirical fact. The subjectivist does this by banking their theory on the feeling of approval, which is a psychological judgment, i.e., it would be a claim about the feelings of the subject, and thus it has truth-value and can be verified. The subjectivist takes statements of the form ‘X is wrong’ and ‘X is right’ as ‘I disapprove of X’ and ‘I approve of X’ respectively. In another post I said that the truth value of “X is wrong/right” on subjectivism would be derived from the individual’s saying it. This is still consistent with what is being said above since the statements ‘I disapprove of X’ is a psychological judgment that is entirely dependent on me. If I were to change my belief and approve of X, then the truth value would change as well. But I digress.
Ayer’s argument against subjectivism is relatively simple and it looks like this: Take the sentence, ‘X is wrong, but I approve of X.’ On the subjectivist view, the first clause ‘X is wrong’ translates to ‘I disapprove of X,’ which renders the original sentence ‘I disapprove of X, but I approve of X,’ as contradictory. Therefore, since subjectivism leads to a contradiction, subjectivism must be false.