ness; and thus the quality of syrup in itself is not different from the quality of vinegar, only the atoms of which each is composed are differently figured and arranged, and hence affect the palate differently. The same explanation would of course apply to the phenomena of the other senses. Different colours are seen because the atoms of light affect the retina differently, some of them impinging on it with greater force and rapidity than others. Such is the manner in which the Atomic philosophers explained the phenomena of sensation and perception. To things themselves they allowed mere quantitative differences, such differences as consist in number, size, figure, motion, weight, and arrangement. These are the only differences which truly exist, which are in rerum natura; because these are the only differences which exist in the atoms of which things are composed. All qualitative differences, such as heat and cold, sweet and bitter, colour and sound—all these they placed in the sentient subject, and regarded as mere affections of the mind or nervous system. Thus the world had reality only in so far as quantity was concerned. In regard to quality, it had no reality out of or beyond the mind of man; and thus, while quantitative difference was real and objective, qualitative difference was only apparent and subjective.
9. It is obvious that this theory of sensation bears a close resemblance to the doctrine frequently propounded in more recent times in regard to the pri-
