conjectured that the transmission of light occupied a certain time: Aristotle declares it to be instantaneous.[199] On passing to terrestrial physics, we find that Aristotle is, as usual, the dupe of superficial appearances, against which other thinkers were on their guard. Seeing that fire always moved up, he assumed that it did so by virtue of a natural tendency towards the circumference of the universe, as opposed to earth, which always moved towards the centre. The atomists erroneously held that all matter gravitated downwards through infinite space, but correctly explained the ascent of heated particles by the pressure of surrounding matter, in accordance, most probably, with the analogy of floating bodies.[200] Chemistry as a science is, of course, an entirely modern creation, but the first approach to it was made by Democritus, while no ancient philosopher stood farther from its essential principles than Aristotle. He analyses bodies, not into their material elements, but into the sensuous qualities, hot and cold, wet and dry, between which he supposes the underlying substance to be perpetually oscillating; a theory which, if it were true, would make any fixed laws of nature impossible. It might have been expected that, on reaching physiology, the Stagirite would stand on firmer ground than any of his contemporaries. Such, however, is not the case. As already observed, his achievements belong entirely to the dominion of anatomy and descriptive zoology. The whole internal economy of the animal body is, according to him, designed for the purpose of creating and moderating the vital heat; and in apportioning their functions to the different organs he is entirely dominated by this fundamental error. It was a common notion among the Greeks, suggested by sufficiently obvious considerations, that the brain is the seat of the psychic activities. These, however, Aristotle transports to the heart, which, in his system, not only propels the blood through the body, but is also the source of heat, the common centre where the different special sensations meet to be compared, and the organ of imagination and of passion. The sole function of the brain is to cool down the blood—a purpose which the lungs also subserve. Some persons believe that air is a kind of food, and is inhaled in order to feed the internal fire; but their theory would involve the absurd consequence that all animals breathe, for all have some heat. Anaxagoras and Diogenes did, indeed, make that assertion, and the latter even went so far as to say that fish breathe with their gills, absorbing the air held in solution by the water passed through them—a misapprehension, says Aristotle, which arose from not having studied the final cause of respiration.[201] His physiological theory of generation is equally unfortunate. In accordance with his metaphysical system, hereafter to be explained, he distinguishes two elements in the reproductive process, of which one, that contributed by