One of the most significant issues in Meta–ethics is moral realism, which has been criticized so much. One of the main criticisms of it has been offered by J.L. Mackie. He believes that moral properties depend upon our feelings and emotions and they don't refer to reality; so, they have no causal explanation and cannot give us any knowledge. Therefore any judgment about moral properties is fundamentally wrong. In this article, at first, Mackie's “error theory” will be considered. Then, considering the conceptual and existential assertions, which are the bedrocks of his idea, and also the “error theory” that John Locke had developed before him, Mackie's theory will be examined. In addition, it will be shown that the subjectivism of moral values faces many different challenges.
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