The complex ideas of substances (man, horse, tree) are formed as follows: The mind observes that a certain number of simple ideas, conveyed in by the different senses, constantly go together, and accustoms itself to regard such a complication of ideas as one object, and designates it by one name. Hence, a substance is nothing but a combination of a certain number of simple ideas, considered as united in one thing. Thus the substance called sun is nothing but the aggregate of the ideas of light, heat, roundness, and constant, regular motion. By substance, the philosophy of the School, and afterwards Descartes, imagined an unknown object, which they assumed to be the support (substratum) of such qualities as are capable of producing simple ideas in us, which qualities are commonly called accidents. But this substance considered as anything else but the combination of these qualities, as something hidden behind them, is a mere phantom of the imagination. We have no distinct idea of such a substratum without qualities. If any one should be asked wherein color or weight inheres, “he would have nothing to say, but the solid extended parts; and if he were demanded what is it that solidity and extension adhere in, he would not be in a much better case than the Indian before mentioned, who, saying that the world was supported by a great elephant, was asked what the elephant rested on; to which his answer was, — a great tortoise; but being again pressed to know what gave support to the broad-backed tortoise, replied, — something, he knew not what.”[#N_26_ (26)] Our knowledge does not extend beyond the assumed accidents, that is, beyond our simple ideas, and whenever metaphysics attempts to proceed beyond them it is confronted with insurmountable difficulties.

John Locke, 26. B. II., chap. XXIII., 2.