An "error theory of ethics" is the view that the ordinary user of moral language is typically making claims that involve a mistake. The concepts of ethics introduce a mistaken, erroneous, way of thinking of the world or of conducting practical reasoning. The theory was most influentially proposed by John L. Mackie in his book Ethics: Inventing Right and Wrong (1977). Mackie believed that ordinary moral claims presuppose that there are objective moral values, but there are no such things. Hence, the practice of morality is founded upon a metaphysical error.

Mackie's arguments against the existence of objective values are of two main kinds. One is the argument from relativity, which cites the familiar phenomenon of ethical disagreement. Another is the argument from "queerness." The moral values whose existence Mackie denies are presented as metaphysically strange facts. They are facts with a peculiar...