Non-Cognitivism is the meta-ethical view (or family of views) that moral utterances lack truth-value (i.e. they are neither true nor false) and do not assert propositions. Therefore, if moral statements cannot be true, and if one cannot know something that is not true, Non-Cognitivism implies that moral knowledge is impossible, and moral truths are not the kind of truths that can be known.

A proposition in Epistemology is, roughly speaking, an assertion or a declarative sentence (as opposed to an interrogative, exclamatory or imperative sentence). Thus, an ethical statement which is a valid proposition (e.g. "Mary is a good person") is able to bear truth values, and one can say of it "that is true" or "that is false". Two people may disagree on its truth or falsity, but it has at least the capacity for truth.

The opposite view to Non-Cognitivism is that of Cognitivism, that ethical sentences express propositions and can therefore be true or false (i.e. they are truth-apt).

Non-Cognitivism is largely supported by the Argument from Queerness: that ethical properties, if they existed, would be different from any other thing in the universe, since they have no observable effect on the world, and there is no way of discerning (and no actual evidence for) the existence of ethical properties . It focuses on the function of normative statements in practice, arguing that they are more likely to merely express approval or disapproval, or to exhort or persuade in a prescriptive way, than to make definitive assertions of truth or falseness. Non-Cognitivists argue that the burden of evidence is on cognitivists who want to show that in addition to expressing disapproval, for example, the claim "Killing is wrong" is also true.

One argument against Non-Cognitivism is that it ignores the external causes of emotional and prescriptive reactions (e.g. if someone says, "John is a good person," then something about John must have inspired that reaction). It is also argued that, if ethical statements do not represent cognitions (as Non-Cognitivism assumes), then how is it possible to use them as premises in an argument, in which they follow the same rules of syllogism as true propositions (e.g. "Killing an innocent human is always wrong. All fetuses are innocent humans. Therefore, killing a fetus is always wrong")?

The following doctrines can be considered Non-Cognitive:

• Emotivism: the view, defended by A.J. Ayer and C. L. Stevenson (1908 - 1979) among others, that ethical sentences serve merely to express emotions of approval or disapproval, and ethical judgements are primarily expressions of one's own attitude, although to some extent they are also imperatives meant to influence or change the attitudes and actions of other listeners.
• Prescriptivism (or Universal Prescriptivism): the view, propounded by R.M. Hare (1919 - 2002), that moral statements function as imperatives which are universalizable (i.e. applicable to everyone in similar circumstances) e.g. "Killing is wrong" really means "Do not kill!"
• Expressivism: the view that the primary function of moral sentences is not to assert any matter of fact, but rather to express an evaluative attitude toward an object of evaluation. Therefore, because the function of moral language is non-descriptive, moral sentences do not have any truth conditions.
• Quasi-Realism: the view, developed from Expressivism and defended by Simon Blackburn (1944 - ), that ethical statements behave linguistically like factual claims, and can be appropriately called "true" or "false" even though there are no ethical facts for them to correspond to. Blackburn argues that ethics cannot be entirely realist, for this would not allow for phenomena such as the gradual development of ethical positions over time or in differing cultural traditions.
• Projectivism: the view that qualities can be attributed to (or "projected" on) an object as if those qualities actually belong to it. Projectivism in Ethics (originally proposed by David Hume and more recently championed by Simon Blackburn) is associated by many with Moral Relativism, and is considered controversial, even though it was philosophical orthodoxy throughout much of the 20th Century.
• Moral Fictionalism: the view that moral statements should not be taken to be literally true, but merely a useful fiction. This has led to charges of individuals claiming to hold attitudes that they do not really have, and therefore are in some way insincere.