The Cambridge History of English and American Literature in 18 Volumes (1907–21). Volume IX. From Steele and Addison to Pope and Swift. XI. Berkeley and Contemporary Philosophy. § 5. Treatise concerning the Principles of Human Knowledge. The Essay did not disclose all that was in Berkeley’s mind. It kept to its topic, the relation of the objects of sight to those of touch, and it did not question the views commonly held about the latter. The full revelation came, a year afterwards, in A Treatise concerning the Principles of Human Knowledge. This small volume, more talked about than read at the time—it took twenty-four years to reach a second edition—is one of the works which have had a critical influence upon the course of European thought. Its importance, in this respect, ranks it with Locke’s Essay and Hume’s Treatise of Human Nature. The fresh step which Berkeley took was short and simple and easy; when taken, it shows us the whole world from a new point of view. Locke had said that all the objects of knowledge are ideas, and he had thus much difficulty—as, indeed, Descartes had had before him—in defending the reality of the things which he supposed to be represented by the ideas. Berkeley solves the difficulty by denying the distinction. The ideas are the things. “It is indeed an opinion strangely prevailing amongst men, that houses, mountains, rivers, and in a word all sensible objects, have an existence, natural or real, distinct from their being perceived by the understanding.” But the opinion needs only to be called in question to show the contradiction it involves; for these objects are the things we perceive by sense, and we perceive nothing but our own ideas. With magnificent confidence, he passes at once to the assertion: Some truths there are so near and obvious to the mind that a man need only open his eyes to see them. Such I take this important one to be, viz. that all the choir of heaven and furniture of the earth, in a word all those bodies which compose the mighty frame of the world, have not any subsistence without a mind; that their being is to be perceived or known.   7