J. L. Mackie selects for critical discussion six related topics which are prominent in John Locke's Essay Concerning Human Understanding: the distinction between primary and secondary qualities; representative theories of perception; substance, real essence, and nominal essence; abstract ideas, universals, and the meaning of general terms; identity, especially personal identity; and the conflict between empiricism and the doctrine of innate ideas. He examines Locke's arguments carefully, but his chief interest is in the problems themselves, which are important for our attempt to decide what sort of world we live in and how we can defend our claim to know about it. The book shows that on most of these topics, views close to Locke's are more defensible than has commonly been supposed, but that there is nonetheless a tension in Locke's thought between extreme empiricism and common-sense or scientific realism. Whereas Locke's immediate successors, Berkeley and Hume, and many later thinkers, have stressed the empiricism at the expense of the realism, this book argues against the more extreme empiricist doctrines but supports the more moderate ones, especially the claims that innate ideas cannot be a source of necessary truth and that authoritative, autonomous knowledge of synthetic truths requires empirical support. The position J. L. Mackie advocates thus reconciles realism with moderate empiricism.