Illustrating Mackie's main argument against the existence of moral facts.
Mackie is arguing a case for metaethical subjectivism or, as he prefers, moral scepticism: his central thesis is that there are no objective moral values. He argues for this in a variety of ways: his main argument is the argument from queerness, though he also utilises an argument from relativity. Though at first compelling, his argument fails, mainly due to the false assumptions upon which they rest.
Mackie argues that moral statements, such as 'Murder is wrong,' do not have a truth-value: they are neither true nor false in the same way that propositions such as 'the curtains are blue,' or 'two plus two equals four,' can be true or false. In other words, he argues that there are no moral facts. He does however recognise that value statements can be propositional if there is an agreed standard to which a something can be compared. For example, the statement 'Lassie is a good sheepdog,' is a value statement but nevertheless does have a truth-value, because there is an agreed standard of what makes a good sheepdog. He also admits that, for example, the standard of justice -- a moral standard -- can make decisions about justice an objective matter. However, he argues that this does not refute his denial of objective values, asserting that it merely shifts the question of objectivity back to the standards themselves. Subjectivism can be contrasted with objectivism, which holds that moral beliefs are matters of fact, but the fact of the matter may depend on situation, and the absolutist view, which claims that there are absolute moral rules such as 'adultery is wrong,' which are true regardless of situation. Mackie's thesis is a metaethical, or second order, claim, rather than a normative, or first order, one: he is not recommending subjectivism as a course of action, instead he is making an observation as to the nature of normative moral statements. He insists that his claim is ontological rather than conceptual or linguistic: he is questioning the existence of objective moral values, rather than analysing the meaning of moral statements or concepts; in this way he differs from emotivists or prescriptivists, both of whom attempt to analyse and give an account for what moral speech actually is, whether expressions of emotion in the former case or prescriptions of advice in the latter.