Unlike his Eleatic teachers, Leucippus was apparently not overly concerned about mixing the ideas of being and not-being, nor about talking about not-being. As far as we know, he did not take the further step, which would soon be taken by Plato, and make gestures at diffusing this worry, by distinguishing between grades of being and types of negation.

The Visible World

In order to account for the phenomena of the observable world, the atomists tell a detailed story about the coming together and separation of atoms in the void. Through their motion, the atoms collide, and though they never really touch, they form objects through their close association. The nature of these objects (and qualities) depends on the variable properties of the atoms thus joined, i.e. their arrangement, size, shape, and motion. Once again, then, what looks like generation, destruction, and change in the observable world, is really not a violation of the Eleatic demands; all that really exists in the most fundamental sense are arrangements of atoms in the void.

Using this theory of atoms in the void, the atomists are the first philosophers to venture a full-fledged theory of sensation. They attempt to explain all of the macroscopic qualities of the world by appealing only to the size, shape, order, and position of atoms.

An excellent example of this attempt is Democritus's account of taste. The sensations of taste, he explains, are entirely a function of the size and shape of atoms in food and their interaction with the atoms of our mouths. Sour taste, he tells us, is the result of angular atoms in twisted configurations. Sweet taste, on the other hand, is caused by rounded atoms of a moderate size. Astringent tastes come from large, barely circular atoms with many angles. Finally, bitter tastes are caused by small, smooth, round atoms, with no hooks on their surfaces. All foods, in fact, have a mixture of all of these sorts of atoms, but it is the predominant sort in the mixture that we perceive most clearly. In effect, what Democritus has done with this account, is to reduce taste to visual and tactile terms. He gives a similarly detailed account of our sensation of color, explaining this phenomenon on the basis of the size and shape of atoms, as well as the nature of the void between them.

Nature of Atomic Motion and Physical Necessity

The atomists give the most detailed account of the motion of their real entities. The atoms, they tell us, move by a jostling, random motion that occurs by collision. Motion, on this view, as on many later views, is transmitted upon collision.

The motion of the atoms is eternal and involves no external forces like love, strife, or mind. Instead the motion, and all else in the physical world, is supposed to be explained by the notion of "necessity." The claim that everything happens by necessity can be seen as a very primitive (and not very well thought out, it seems) form of modern determinism—the view that every event is an effect of some prior series of effects. The order of the cosmos, then, is not imposed by some outside force on the atomist view. In modern terms we would say that order prevails in the atomist view because it falls out of the laws of nature, which govern the atoms. (Another way to put this is to say that the ultimate controlling principle in nature is that everything follows the laws of its own being.) But the atomist notion of necessity probably did not take this sophisticated a form, drawing on the concept of natural law. A more accurate description of their notion of necessity would only assert that X determines some future Y because X has the proper atoms, plus the suitable motion, to yield Y and only Y. This form of determinism is very weak, so weak that it cannot really be made to work; there is nothing else in system to explain why X would determine Y and only Y, since there is no idea of natural law.