Unlike the useless substance of the Indians, the substance of Democritus could offer a calculable ground for the flux of appearances; for this substance was distributed unequally in the void, and was constantly moving. Every appearance, however fleeting, corresponded to a precise configuration of substance; it arose with that configuration and perished with it. This substance, accordingly, was physical, not metaphysical. It was no dialectical term, but a scientific anticipation, a prophecy as to what an observer who should be properly equipped would discover in the interior of bodies. Materialism is not a system of metaphysics; it is a speculation in chemistry and physiology, to the effect that, if analysis could go deep enough, it would find that all substance was homogeneous, and that all motion was regular. Though matter was homogeneous, the forms of the ultimate particles, according to Democritus, were various; and sundry combinations of them constituted the sundry objects in nature. Motion was not, as the vulgar (and Aristotle) supposed, unnatural, and produced magically by some moral cause; it had been eternal and was native to the atoms. On striking, they rebounded; and the mechanical currents or vortices which these contacts occasioned formed a multitude of stellar systems, called worlds, with which infinite space was studded. Mechanism as to motion, atomism as to structure, materialism as to substance, that is the whole system of Democritus. It is as wonderful in its insight, in its sense for the ideal demands of method and understanding, as it is strange and audacious in its simplicity. Only the most convinced rationalist, the boldest prophet, could embrace it dogmatically; yet time has largely given it the proof. If Democritus could look down upon the present state of science, he would laugh, as he was in the habit of doing, partly at the confirmation we can furnish to portions of his philosophy, and partly at our stupidity that cannot guess the rest. There are two maxims in Lucretius that suffice, even to this day, to distinguish a thinker who is a naturalist from one who is not. “Nothing,” he says, “arises in the body in order that we may use it, but what arises brings forth its use.”[1] This is that discarding of final causes on which all progress in science depends. The other maxim runs: “One thing will grow plain when compared with another: and blind night shall not obliterate the path for thee, before thou hast thoroughly scanned the ultimate things of nature; so much will things throw light on things.”[2] Nature is her own standard; and if she seems to us unnatural, there is no hope for our minds.