Berkeley is at once a theorist of knowledge and a metaphysician, combining, in a way, the method of Locke with the method of Descartes and his successors. The popular notion of his philosophy is that it resolved the external world into a dream, or at least into something that has no existence outside our minds. But this is an utter misconception, against which Berkeley constantly protested. His quarrel was not with common sense, but with the theorists of perception. To understand this we must return for a moment to Locke's teaching. It will be remembered in what a tangle of difficulties the essay had left its author. Matter had two sets of qualities, primary and secondary, the one belonging to things in themselves, the other existing only {73} in our minds; yet both somehow combined in real substances independent of us, but acting on our senses. Substance as such is an unknown and unknowable postulate; nevertheless, we know that it was created by God, of whom our knowledge is, if anything, inconveniently extensive. Now Berkeley, to find his way out of these perplexities, begins by attacking the distinction between primary and secondary qualities. For this purpose his _Theory of Vision_ was written. It proves--or attempts to prove--that extension is not a real attribute of things in themselves, but an intellectual construction, or what Locke would have called an "idea of reflection." Till then people had thought that its objectivity was firmly established by the concurrent testimony of two senses, sight and touch. Berkeley shows, on the contrary, that visible and tangible extension are not the same thing, that the sensations--or, as he calls them, the ideas--of sight and touch are two different languages whose words we learn by experience to interpret in terms of each other without their being necessarily connected. A man born blind would not at first sight know how to interpret the visual signs of distance, direction, and magnitude; he would have to learn them by experience. These, in fact, are ideal relations only existing in the mind; and so we have no right to oppose mind as inextended to an extended or an external world.