If you had asked me a couple of weeks ago, I would have guessed that the most fundamental distinction in metaethics is the one between moral realists and moral anti-realists. Realists hold that moral facts are mind-independent facts, objective properties about the real world. Anti-realists, on the contrary, think moral facts are subjective, or are matters of convention among a group of people.
There is, however, a still more fundamental distinction; between the (unhelpfully named) cognitivist and non-cognitivist approaches to ethics. Cognitivists hold that moral claims, say, "Eating meat is wrong," are propositions: statements that might be true or might be false, but about which it is meaningful to argue whether they are true or false. For some cognitivists (anti-realists), the truth or falsity might be a matter of subjective opinion or group convention; for others (realists), it might be a matter of objective fact. But for both, moral statements can be meaningfully given the labels "true" or "false," at least with respect to a person's beliefs.
Non-cognitivists, then, deny that moral claims can be true or false, or that they say anything about anyone's beliefs. Rather, moral statements express opinions, or are prescriptive. On the former view (known as emotivism), "Eating meat is wrong" is simply a way of saying "I disapprove of eating meat - and you should, too!" Or, as it is sometimes put more briefly, "Boo on eating meat!" The prescriptivist view says something similar, but emphasizes the "you should, too!" part: moral statements are imperatives; commands that should be obeyed, not just by the speaker, but by everyone. Non-cognitivists are necessarily moral anti-realists.
Non-cognitivism is similar to some anti-realist cognitive accounts, as the Stanford Encyclopedia of Philosophy points out:
It is useful to contrast non-cognitivism with one particular variety of cognitivism in order to more clearly present what the non-cognitivist is claiming. Various versions of cognitivist subjectivism equate moral properties such as rightness with the property of being approved of by some person or group. To be right is to be approved of by the speaker, or the speaker and her friends, or the members of the speaker's society, or everybody...
But this by itself is not sufficient to make the position non-cognitivist. This variety of subjectivism agrees with one of the positive non-cognitivist theses (that moral utterances conventionally express non-cognitive attitudes), but it does not agree with either of the essential negative non-cognitivist claims (that the judgments don't express beliefs and/or that they are not truth-apt)....
A simple example gets the idea across. One can express dislike of something by saying that one dislikes it. This is the way that a cognitivist subjectivist thinks we express moral attitudes. But one can also express dislike of something by booing or hissing. This is much like the way some non-cognitivists think we express moral attitudes. The latter way of expressing an attitude is different from the way cognitivist subjectivists think we express moral attitudes because it expresses the attitude without saying that we have the attitude.
Thus, for the non-cognitivist, moral statements are not claims about the beliefs or values of the speaker or of some group, they are merely speech acts that express approval, or disapproval, or that encourage conformity with some norm.
Problems arise for the non-cognitivist when simple moral statements are part of longer sentences. From the Wiki page:
Attempts to translate these sentences in an emotivist framework seem to fail (e.g. "She does not realize, 'Boo on eating meat!'"). Prescriptivist translations fare only slightly better ("She does not realize that she is not to eat meat").
- Eating meat is not wrong.
- Is eating meat wrong?
- I think that eating meat is wrong.
- Mike doesn't think that eating meat is wrong.
- I once thought that eating meat was wrong.
- She does not realize that eating meat is wrong.
This is called the embedding problem. It seems that, at least in the way they are normally used, moral claims express more than mere (dis)approval.