Locke had treated ideas of sensation as things which mediate between the knowing mind and corresponding material objects which they resemble where there is real knowledge ; but Berkeley denounces this distinction between ideas and external objects as "the very root of scepticism"—"for how can it be known that the things which are perceived are conformable to those which are not perceived?" (Principles, 86). For Berkeley the ideas are the objects of knowledge, and there is nothing beyond them. "All the choir of heaven and furniture of earth, in a word all those bodies which compose the mighty frame of the world, have not any substance without the mind; . . . their being is to be per ceived or known . . . consequently so long as they are not actually perceived by me, or do not exist in my mind, or that of any other created spirit, they must either have no existence at all, or else subsist in the mind of some Eternal Spirit" (Ibid. § 6). With the substantiality of matter its causality is also rejected. One idea of sensation cannot cause another; it can only suggest it. "The fire which I see is not the cause of the pain I suffer upon my approaching it, but the mark that forewarns me of it" (Ibid. § 65). And if one asks how it comes about that there appear to be such regular connections between divers ideas, the answer is "that this is done in virtue of an arbitrary connection instituted by the Author of Nature" (Theory of Visual Language, § 43). For spirits or minds are active agents, in fact the only ones.
Mechanistic explanations are therefore illusory. And if one objects (as Hume did soon afterwards) that there seems to be no more reason for assuming a mental substance than a material substance, ideas being the only things known, then Berkeley's reply is as follows : "I myself am not ideas, but . . . a thinking, active principle that . . . operates about ideas. I know that I . . . perceive both colours and sounds: that a colour cannot per ceive a sound, nor a sound a colour : that I am therefore one in dividual principle, distinct from colour and sound, and . . . all other sensible things or inert ideas" (Third Dialogue). He admits, however, that he has no "idea" of minds or spirits, only a "notion." Anyway the net result of Berkeley's speculations is an idealist philosophy according to which the only realities are God, other spirits or minds which He has created, and the innumerable ideas which He has produced and arranged for us to apprehend in certain sequences arbitrarily decreed by Him. (See BERKELEY;
IDEALISM; KNOWLEDGE, THEORY OF.) Hume (1711-1776) pursued the problems and methods of Locke to their extreme conclusion, in the sense that he showed that the kind of empiricism which Locke had advocated leads to positivism in science and scepticism in philosophy. It is one of the ironies of history that the book which Berkeley wrote in order to prevent or to cure scepticism actually infected Hume with it. "He professes [so writes Hume of Berkeley] in his title-page . . . to have composed his book against the sceptics as well as the atheists and freethinkers. But that all his arguments, tho' otherwise intended, are in reality merely sceptical, appears from this, that they admit of no answer and produce no conviction. Their only effect is to cause that momentary amazement and ir resolution and confusion, which is the result of scepticism" (Enquiry Concerning Human Understanding, section 12, part i.n.). Berkeley had contended that there is not sufficient evi dence for assuming material substances or material causality, as we have no ideas of either ; but he defended both the sub stantial nature and causal power of minds or spirits. Hume ar gued that the same reasons which led Berkeley to reject material substances and material causes are also valid against the assump tion of mental substances and mental causes.
Berkeley had tried to save mental substances and mental agency by pleading that we have "notions" of them apprehended by rea son, though not "ideas" of experience. But Hume rejected this view of a radical distinction between "reason" and "experience," and submitted everything, even rational judgments, to the test of experience, and insisted on accounting for everything in terms of experience. He accordingly denied the certainty even of so called mathematical knowledge, and reduced all alleged knowledge or certainty to mere probability. Causality itself he explained away as habitual association of sequent impressions or ideas; and substances as due to the mistaking of similar, recurring im pressions for continuous impressions of the same thing. (See