SOURCES OF HUME'S SCEPTICAL

PHILOSOPHY.

The Treatise of Human Nature,' Hume's first and greatest work, is connected in the closest possible way with the systems of Locke and Berkeley.

(1) Locke, in trying to show that all knowledge depends upon experience, had thought it necessary to prove that all ideas, the elements of knowledge, are derived from experience. He succeeded in doing this to his own satisfaction, but only because he failed to distinguish between pure sensations and their revived images in memory and imagination on the one hand, and these sense-images together with the closely associated intellectual factors which enter into the simplest act of knowledge on the other. For example, he said that the idea of impenetrability is derived from the sense of touch, and that if any one desires to ascertain the content of this idea he may "put a flint or a football between his hands and then endeavor to join them, and he will know." Here Locke did not distinguish from the mere muscular and tactual sensations involved, the additional complex thought that in spite of the effort made it is impossible to bring the hands together, because there

is something between them that resists extinction. Yet it is clear that this thought is not a part of the sensations involved, and that without it we could have no idea of impenetrability.

(2) Berkeley accepted Locke's conclusion that all the elements of knowledge are derived from senseexperience, but he saw as Locke did not, that sensations and their fainter reproductions consist simply of images presented to some sense or other-of visual, auditory, or tactual pictures, as it were. Berkeley therefore supposed that all thought consists of nothing but a series of simple or complex images.

(3) But every image is an image, not of a so-called general idea, but of some particular thing, more or less definitely conceived. We cannot, for example, picture a triangle which is not either equilateral, isosceles, or scalene, nor imagine a taste which is neither sweet, sour, saline, or the like. There are, therefore, no abstract ideas, or ideas of things or qualities in general.

(4) One idea especially, of which Locke spoke, Berkeley could not picture: that, namely, of an inert, senseless something called substance, which has all the qualities perceived by the senses but is not any of them. So he concluded that the only possible idea of substance is the complex of ideas of the individual qualities of a particular object as they present themselves to the human mind through the organs of sense; and that, as the mind knows only these ideas, it is illogical, unnecessary, and even absurd to assert the existence of an absolutely unknown something called substance, or matter, to account for these sensations.

These four conclusions reached by Locke and Berkeley-that all ideas are derived from experience, that experience is only of individual mental images, and that therefore there can be no abstract ideas, and no idea of a substance which underlies the perceptible qualities of things-these are the whole basis on which Hume's system rests.