This way of regarding the popular notions of weight and lightness is clearly formulated for the first time in Plato’s _Timaeus_.[960] There is no such thing in the world, we are told there, as “up” or “down.” The middle of the world is not “down” but “just in the middle,” and there is no reason why any point in the circumference should be said to be “above” or “below” another. It is really the tendency of bodies towards their kin that makes us call a falling body heavy and the place to which it falls “below.” Here Plato is really giving the view which was taken more or less consciously by his predecessors, and it is not till the time of Aristotle that it is questioned.[961] For reasons which do not concern us here, he definitely identified the circumference of the heavens with “up” and the middle of the world with “down,” and equipped the four elements with natural weight and lightness that they might perform their rectilinear motions between them. As, however, Aristotle believed there was only one world, and as he did not ascribe weight to the heavens proper, the effect of this reactionary theory upon his cosmical system was not great; it was only when Epicurus tried to combine it with the infinite void that its true character emerged. It seems to me that the nightmare of Epicurean atomism can only be explained on the assumption that an Aristotelian doctrine was violently adapted to a theory which really excluded it.[962] It is totally unlike anything we meet with in earlier days. Footnote 960: Plato, _Tim._ 61 c 3 sqq. Footnote 961: Zeller says (p. 876) that in antiquity no one ever understood by weight anything else than the property of bodies in virtue of which they move downwards; except that in such systems as represent all forms of matter as contained in a sphere, “above” is identified with the circumference and “below” with the centre. As to that, I can only say that no such theory of weight is to be found in the fragments of the early philosophers or is anywhere ascribed to them, while Plato expressly denies it. This brief historical survey suggests at once that it is only in the vortex that the atoms acquire weight and lightness,[963] which are, after all, only popular names for facts which can be further analysed. We are told that Leukippos held that one effect of the vortex was that like atoms were brought together with their likes.[964] In this way of speaking we seem to see the influence of Empedokles, though the “likeness” is of another kind. It is the finer atoms that are forced to the circumference, while the larger tend to the centre. We may express that by saying that the larger are heavy and the smaller light, and this will amply account for everything Aristotle and Theophrastos say; for there is no passage where the atoms outside the vortex are distinctly said to be heavy or light.[965] Footnote 962: