Non-cognitivism is the meta-ethical view that ethical sentences do not express propositions and thus cannot be true or false (they are not truth-apt). A noncognitivist denies the cognitivist claim that "moral judgments are capable of being objectively true, because they describe some feature of the world." If moral statements cannot be true, and if one cannot know something that is not true, noncognitivism implies that moral knowledge is impossible.
Non-cognitivism entails that non-cognitive attitudes underlie moral discourse and this discourse therefore consists of non-declarative speech acts, although accepting that its surface features may consistently and efficiently work as if moral discourse were cognitive. The point of interpreting moral claims as non-declarative speech acts is to explain what moral claims mean if they are neither true nor false (as philosophies such as logical positivism entail). Utterances like "Boo to killing!" and "Don't kill" are not candidates for truth or falsity, but have non-cognitive meaning.
Varieties of non-cognitivism
Emotivism, associated with A. J. Ayer, the Vienna Circle and C. L. Stevenson, suggests that ethical sentences are primarily emotional expressions of one's own attitudes and are intended to influence the actions of the listener. Under this view, "Killing is wrong" is translated as "Killing, boo!" or "I disapprove of killing; do so as well."
A close cousin of emotivism, developed by R. M. Hare, is called universal prescriptivism. Prescriptivists interpret ethical statements as being universal imperatives, prescribing behavior for all to follow. According to prescriptivism, phrases like "Thou shalt not murder!" or "Do not steal!" are the clearest expressions of morality, while reformulations like "Killing is wrong" tend to obscure the meaning of moral sentences.
Other forms of non-cognitivism include Simon Blackburn's quasi-realism and Allan Gibbard's norm-expressivism.
Arguments in favour of non-cognitivism
Arguments for prescriptivism focus on the function of normative statements.
Prescriptivists argue that factual statements and prescriptions are totally different, because of different expectations of change in cases of a clash between word and sentence. In a descriptive sentence, if one premises, that "red is a number," according to the lexical rules of English grammar, then said statement would be false. Since said premise describes the objects; red and number, anyone with an adequate understanding of English would notice the falseness of such description and the falseness of said statement. However, if the norm "Thou shallt not Kill!" is uttered, and this premise is negated (by the fact of a person being murdered), the speaker is not to change his sentence upon observation of this into: "Kill other people!", but is to reiterate the moral outrage of the act of killing. Adjusting statements based upon objective reality and adjusting reality based upon statements are contrary uses of language, so descriptive statement are a different kind of sentences than norms. If truth is understood according to correspondence theory, the question of the truth or falsity of sentences not contingent upon external phenomena cannot be tested (see tautologies ).
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