and number, and arrangement of the insensible atoms of which these things are composed,—in other words, mere qualitative differences. For example, if you ask, Why is water soft and flowing? the answer would be, that the minute atoms of which it consists are smooth and round, and do not fit into each other—like small wheels or globes, they roll over each other, hence its yielding and fluid nature. Why, again, is iron fixed and unyielding? The answer is, because its minute and insensible particles are not smooth and round, but are jagged and uneven—have, as it were, teeth by which they cling to each other, and, thus cohering, form a compact and solid body. But in both cases the atoms are in themselves of the same quality; they are merely different in shape, size, arrangement, and these are not qualitative but quantitative differences. In short, there are in reality no differences in the universe, except differences of quantity. All qualitative differences are unreal, and are merely apparent. So much, then, for the way in which the Atomic philosophy simplifies, or aims at simplifying, whether successfully or not, the theory of the universe, by abolishing quality, and by reducing all the diversities of natural agents to a difference of quantity merely.
8. The second point of interest in the Atomic philosophy is the new theory of sensation and perception which it involved. It had hitherto been supposed that there were certain qualities in objects corre-
