According to John Stuart Mill, George Berkeley made "three first-rate philosophical discoveries, each sufficient to have constituted a revolution in psychology, and which by their combination have determined the whole course of subsequent philosophical speculation." Mill was praising Berkeley's theory of vision, critique of abstractions, and recognition that sensations provide humanity's only knowledge of reality. The last of these influenced David Hume and, through him, Immanuel Kant. These three discoveries guarantee Berkeley's place in the history of philosophy. Beyond it, however, his name appears widely, from William Butler Yeats's poetry to discussions of quantum mechanics and of Asian religion (such as the Mahayana Buddhist doctrine that all is mind). This greater renown comes for the controversial core of his philosophy: an attack on materialism so thorough that he denied even the existence of matter. Consequently, he has been considered a precursor by many who, for whatever reason, challenge...