"The mind being, as I have declared, furnished with a great number of the simple ideas, conveyed in by the senses, as they are found in exterior things, or by reflexion on its own operations, takes notice also, that a certain number of these simple ideas go constantly together; which, being presumed to belong to one thing, and words being suited to common apprehensions, and made use of for quick despatch, are called, so united in one subject, by one name; which, by inadvertency, we are apt afterward to talk of, and consider as one simple idea, which indeed is a complication of many ideas together: because, as I have said, not imagining how these simple ideas can subsist by themselves, we accustom ourselves to suppose some _substratum_ wherein they do subsist, and from which they do result, and which therefore we call _substance_." It is clear enough from this, as it is clear enough from other passages in the same book, that Locke talked as though the complex of simple ideas in consciousness were the very same thing (in Sense I) as the group of "real" qualities outside of consciousness. And no careful reader of his book can avoid seeing that the confusion of his language is a fair index to the confusion of his thoughts with regard to the two.[7] It is little better in the case of "bodies" and substance. In the passage just given, he would seem to make _substance_ an obscure something underlying groups of ideas, and not groups of real qualities, but in the next sentence he makes it a substratum of the qualities which produce in us ideas. In many passages[8] he distinguishes between _substance_ and _substances_, by which latter he means groups of "real" qualities with the added substratum or substance; as such substances he instances oak, elephant, iron. He emphasizes the fact that substance is not to be confounded with substances, which are things of different sorts.[9] In so far as the substances are bundles of qualities, they are known to us through sensation.[10] In so far as they are also substance, they cannot be known to us through sensation, for the idea of substance is not known through sensation.[11] We are not then to look upon substance as such a constituent part of "a substance" as a quality is. The two belong to different classes. And if we offer proof for the existence of "real" substances, which is evidently applicable to them only as bundles of qualities--proof from sensation--then substance is overlooked altogether. It is significant that Locke, having thus put together under one head as "a substance" an oak viewed as a bundle of "real" qualities and an oak viewed as substratum, proceeded to argue as if he had but one thing to prove when he felt called upon to defend the real existence of substances. In his chapters on "The Extent of Human Knowledge," the "Reality of Knowledge," and "Our Knowledge of the Existence of Other Things,"[12] he devotes himself wholly to proving things as bundles of qualities, and pays no