At the opposite pole from immediacy lies intelligibility. To reduce phenomena to constant elements, as similar and simple as possible, and to conceive their union and separation to obey constant laws, is what a natural philosopher will inevitably do so soon as his interest is not merely to utter experience but to understand it. Democritus brought this scientific ideal to its ultimate expression. By including psychic existence in his atomic system, he indicated a problem which natural science has since practically abandoned but which it may some day be compelled to take up. The atoms of Democritus seem to us gross, even for chemistry, and their quality would have to undergo great transformation if they were to support intelligibly psychic being as well; but that very grossness and false simplicity had its merits, and science must be for ever grateful to the man who at its inception could so clearly formulate its mechanical ideal. That the world is not so intelligible as we could wish is not to be wondered at. In other respects also it fails to respond to our ideals; yet our hope must be to find it more propitious to the intellect as well as to all the arts in proportion as we learn better how to live in it. The atoms of what we call hydrogen or oxygen may well turn out to be worlds, as the stars are which make atoms for astronomy. Their inner organisation might be negligible on our rude plane of being; did it disclose itself, however, it would be intelligible in its turn only if constant parts and constant laws were discernible within each system. So that while atomism at a given level may not be a final or metaphysical truth, it will describe, on every level, the practical and efficacious structure of the world. We owe to Democritus this ideal of practical intelligibility; and he is accordingly an eternal spokesman of reason. His system, long buried with other glories of the world, has been partly revived; and although it cannot be verified in haste, for it represents an ultimate ideal, every advance in science reconstitutes it in some particular. Mechanism is not one principle of explanation among others. In natural philosophy, where to explain means to discover origins, transmutations, and laws, mechanism is explanation itself. Heraclitus had the good fortune of having his physics absorbed by Plato. It is a pity that Democritus’ physics was not absorbed by Aristotle. For with the flux observed, and mechanism conceived to explain it, the theory of existence is complete; and had a complete physical theory been incorporated into the Socratic philosophy, wisdom would have lacked none of its parts. Democritus, however, appeared too late, when ideal science had overrun the whole field and initiated a verbal and dialectical physics; so that Aristotle, for all his scientific temper and studies, built his natural philosophy on a lamentable misunderstanding, and condemned thought to confusion for two thousand years.