Berkeley is well known for his theory of vision that contributed so much to modern psychology of perception. However, in that theory he completely repudiated the primary bastion of empiricism: namely, matter. Berkeley departed from both common sense and science in elaborately insisting that matter—the entire physical world—based on our best evidence, simply did not exist in the way that the other empiricists—Hobbes, Locke and Hume, and later on, John Stuart Mill and Bertrand Russell—assumed that it did. For any serious student of the history of philosophy, Berkeley is either a delightful aberration or an intractable obstacle because of this position.