FROM LOCKE TO HTJME Locke (1632-1704) has the distinction of having given a de cidedly epistemological turn to modern philosophy. He relates that five or six friends used to discuss philosophical or theological problems with him, but fruitlessly. So it occurred to him "that before we set ourselves upon inquiries of that nature, it was necessary to examine our own abilities, and see what objects our understandings were or were not fitted to deal with." By "extend ing their inquiries beyond their capacities" people only "raise questions and multiply disputes, which, never coming to any clear resolution, are proper only to continue and increase their doubts, and confirm them at last in perfect scepticism." Locke attempted to carry out the task which he set himself by means of a de scriptive account of human "ideas" or experiences, of which he drew up a detailed inventory in his Essay concerning Human Understanding (169o). Locke uses the term "idea" for any "object of the understanding," any object as experienced; and, directly or indirectly, he traces all ideas to experience. This con stitutes his empiricism. He rejects innate ideas, maintaining that prior to experience the mind is like a blank paper ; but he credits the mind with certain powers. Ideas are of two kinds in respect of their origin. Some are induced by sensible objects— they are "ideas of sensation." Others result from reflection upon such ideas of sensation—they are "ideas of reflection." And the mind not only reflects, but also combines the given simple ideas into complex ones.
Locke believes that there are external things. But the mind does not apprehend them directly ; ideas of sensation are at best only "appearances" of things, only copies of them, mediating between them and the apprehending mind. And this is true only of ideas relating to primary qualities (extension, figure, number, motion, solidity) ; the ideas of sensation relating to secondary qualities (colour, smell, etc.) are simply secondary or subjective effects produced in us by the primary qualities, and are not copies of anything objective. By combining the simple ideas of
sensation and reflection there result complex ideas of substances, modes (i.e., states of substances), and relations. The idea of a substance, or substratum, supporting qualities, etc., correspond ing to simple ideas, Locke regards as an invention—he cannot trace it to a simple idea of experience, and he admits that it is vague. Yet he accepts the reality of substances, both physical and mental substances.
Of his own existence, each man is absolutely sure, by intuition; he cannot even doubt it without presupposing it. Of God's exist ence we are also certain, by demonstration; for our own existence cannot be explained without reference to Him as its cause. But, all the same, the "real essence" of substances is unknowable, and Locke is "apt to doubt a science of physical bodies as out of our reach" beyond merely empirical limits. (See LOCKE; EMPIRICISM; KNOWLEDGE, THEORY OF ; PSYCHOLOGY, HISTORY OF.) Berkeley (1685-1753) set himself the task of opposing the mechanistic methods of explanation generally accepted in his time in consequence of the fashion set by the great pioneers of modern science. He feared that that way lay materialism and atheism. But it was Locke's Essay that served him chiefly as the text of his criticism, especially in his Principles of Human Knowledge (1710) and the Three Dialogues (1713). Locke had maintained that our ideas of primary qualities resemble their external objects, whereas those of secondary qualities have no corresponding objects. Berkeley objected that both kinds of ideas are equally dependent on the mind and there is no more need or justification to assume the objective existence of primary than of secondary qualities. Moreover it is absurd to suppose that an idea can resemble anything that is not an idea. And if it is superfluous to assume the objective existence of primary qualities corre sponding to certain ideas of sensation, it is even more unnecessary to assume, with Locke, the independent existence of material substance, of which, strictly speaking, we have no idea at all.