But the effluxes vary very much in the degree of fineness of their atomic structure. There are all sorts, from very fine to very coarse. Since the efflux must correspond to a particular sense if that sense is to be affected by it, the effluxes that can affect the senses vary respectively as to their fineness. Democritus was particularly interested in the sensations of sight and hearing as examples of this. None of the effluxes affecting the senses are as fine as those that stimulate the reason. Unless they were the finest of all the effluxes, they could not affect the fine motions of the fire atoms of the reason. These finest “eidola” or effluxes are the true copies of things, and the reason therefore alone knows things truly. Thought, on the one hand, is precisely the atomic motion of the _direct_ impact of the finest effluxes upon the fine fire-atoms of the soul. Sensation, on the other hand, is atomic motion from the _indirect_ impact of the coarser grades of effluxes upon the fire atoms. The reason knows reality directly. Sensations are aroused in a roundabout way by the coarse effluxes setting in motion the corresponding sense organ, which in turn sets in motion the fire atoms. Thus does Democritus make the distinction between thought and sensation in quantitative terms. Thus does he reduce his psychology to a consistency with his metaphysical principle of materialism. =Democritus’ Theory of Knowledge――The World of Twofold Reality.= Democritus would have been only one of the great Cosmologists, and he would not have his place by the side of Plato and Aristotle, if his materialism had illuminated no other subject than physics. Indeed, it is doubtful if his physics would have been so grandly comprehensive and unqualified had it not been strengthened by his discriminating theory of twofold knowledge. He might have extended and systematized his materialism so that it explained to the satisfaction of his time both physical and psychical phenomena, and still have been a hylozoist, like Leucippus, the founder of the Atomistic school. The problem of knowledge――the problem of estimating our mental states――was as incomprehensible to Leucippus as to the Eleatics. Democritus, however, was a rationalist and realist like Plato and Aristotle. He recognized, as did they, that there is a difference in epistemological values. His universalized materialism did not prevent him from evaluating our experiences from the same general point of view as the leader of the Academy and the Stagirite. He felt that a twofold reality is as consistent with materialistic principles as with idealism. So he reduced all qualities to quantities, and then as quantities re-valued and classified them. His chief contribution was to the subject of epistemology and not to physics, and that is why he is treated among the Greek Systematizers and not among the Cosmologists. Probably his chief interest lay where he did his chief work.