Just here is where the weakness in the atomic theory is introduced by Epicurus. Democritus accounted for the collision and rebound of atoms under law, by a _cause_, not by chance. Epicurus threw away the cause and introduced “chance collisions.” But no atom was ever deviated from its flight in parallel lines by “chance”--in this case a wholly metaphysical invention. No worlds, no suns or planets could be built up on the theory of particles falling through infinite space in parallel rectilinear paths. There must be collision and recoil of atoms in order that the lateral and whirling movements, or vortices, should be set up, which were supposed to cause the atoms to combine according to their shape, size, and weight, the heavier forming the denser portions of the earth, the lighter constituting the atmosphere. This difficulty did not escape the astute mind of Lucretius. But in attempting to solve what is really an insoluble problem--the development of worlds under the conditions stated--the great Roman poet only made bad matters worse. For here he introduced the dogma of free will applied to the falling atoms. They were made to deviate from their parallel lines by a voluntary movement! And it appears that this was done on the authority of Epicurus himself: “In these errors,” says Lange, “Democritus probably had no share; and yet we shall judge them more leniently if we reflect that, even to our own day, the essence of the doctrine of the freedom of the will, with whatever metaphysical subtlety it is elaborated, consists simply of the uncertainty and perplexity of phenomenal appearances.” Again: “Whilst, therefore, it is one of the most important efforts of recent materialism to deduce the whole mass of voluntary movements from mechanical causes, we find Epicurus adopting a quite incalculable element into his system.”[17] For Democritus a “chance” occurrence would be a causeless event, and this he could not allow under his law of “necessity.” An efficient cause produces always appropriate and calculable results under given conditions, and these results are always uniform, predictable occurrences, given knowledge of the mechanical factors involved. To the earlier Atomists it seemed clear that particles which are alike in weight and in form would “naturally,” when whirled about in a vortex, be brought to the same location; and because of their peculiar shape, become entangled one with another. Since some atoms were supplied with hooks or even with balls and sockets, some with mortice or dovetail, others with sharp, rough or involuted edges, what more “natural” than that groups or combinations of atoms should be formed, even to the cohesion of some atoms in places not suitable to their nature, resulting in the formation of compound bodies?