What kind of properties are moral qualities, such as rightness, badness, etc? Some ethicists doubt that there are any such properties; they maintain that thinking that something is morally wrong (for example) is comparable to thinking that something is a unicorn or a ghost. These moral error theorists argue that the world simply does not contain the kind of properties or objects necessary to render our moral judgments true. This radical form of moral skepticism was championed by the philosopher John Mackie (1917-1981). This anthology is a collection of philosophical essays critically examining Mackie's view.