Non-cognitivism
Non-cognitivism is a fringe meta-ethical view that ethical sentences (e.g. "Killing is wrong") do not
- express propositions
- convey factual beliefs
- have cognitive meaning
- have a truth value (i.e. it makes no sense to say they are true or false)
- make claims about the world
This is in constrast to statements such as "The sky is blue", which do do the above things.
The Claim
Note that non-cognitivism is not saying that whether ethical sentences are true or false is "subjective" (compare Ethical subjectivism), or that all ethical sentences are false (compare Moral skepticism). True and false simply don't apply to ethical sentences.
Nonetheless, non-cognitivists insist that moral sentences do serve a purpose. Different schools within non-cognitivism have different ideas about what that purpose might be. Emotivists say that ethical sentences express and evoke certain kinds of emotion regarding what appears to be the sentence's cognitive content. Thus saying "Kindness is good" would not very different from yelling "Hurray for kindness!" Alternatively, saying "Murder is bad" would have a somewhat similar effect to first saying the word "murder" and then booing in the manner of a sports fan.
It is easy to compare emotivism with some versions of ethical subjectivism. Consider the case of "Kindness is good". Instead of taking this as akin to "Hurray for kindness!", some moral subjectivists might take this as akin to "I strongly approve of kindness." Although these two paraphrases may seem similar, the emotivist will claim that three differences are critical: First, only the latter is a literal statement about the speaker's mental state. Second, only the latter can be literally true or false. Third, there may be a situation or twi where the former cannot be substituted for the latter. This is why the theory is a fringe theory.
Prescriptivists, on the other hand, say that ethical sentences are implicit commands or recommendations. "Kindness is good" obviously means something like "Be kind" or "I suggest you be kind."