Like Anaxagoras and Empedocles, the atomists wanted to answer the basic post- Eleatic question: if change cannot occur in the real, then how does it occur in the observable world? Also like the previous two philosophers, they answered this question by postulating the existence of certain elements of the cosmos that are real in the Parmenidean sense and by claiming further that through analyzing the arrangement and rearrangement of these basic elements, we can arrive at an account of the visible world without having to admit that there is any change on the level of the real. But whereas the two previous pluralists rejected the Eleatic notion that what exists is one in kind, the atomists retained this contraint. The atomists posit just one kind of real thing — tiny, indivisible atoms, swimming around in a void. This account of reality is by far most sophisticated of all those ventured by the Presocratics, and it even comes alarmingly close to anticipating the modern scientific view of ultimate reality.
The only two known Presocratic atomists were Leucippus and his student Democritus. Unfortunately, we know very little about Leucippus, the founder of atomic theory. Even his place of birth is in dispute, given variously as Miletus, Abdera, and Elea. What we do know with moderate certainty is that Leucippus studied with members of the school of Elea at some point in his life. He was clearly influenced by Zeno as is evidenced by his strong interest in the problems and paradoxes of space. The only other fact we know about this great thinker is that he wrote two books, no parts of which survive. The first of these was called On Mind and the second The Great World System.
Democritus was the student of Leucippus, and he is the figure through whom atomism has been transmitted to later generations. It is not known how much of his theory is simply a repetition of Leucippus's teaching and how much of it is original to him, but it was he who brought atomism to public attention and who made it a matter of philosophical controversy. He born around 460 B.C. in Abdera, Thrace in Northern Greece, and he traveled throughout the ancient world. We are aware of the titles of at least seventy books that he supposedly authored, and these works cover a wide variety of subjects. He wrote in nearly all philosophical areas, including mathematics, natural philosophy, literature, and grammar, and also wrote more popular works, such as accounts of his travels. In addition, he seems to have written on farming, medicine, military science, and even painting. Interestingly, not only did he have something of worth to say on all of these topics, but he even applied atomic theory to most of them. He apparently believed that atomism could be usefully extended to all aspects of the world, including even ethics and politics.
Like Anaxagoras and Empedocles, The Atomists claimed that there was a level of reality that satisfied the Eleatic demands. This level of reality was populated by atoms and the void. Atoms are, literally, indivisible particles, which are so small that they can be split no further. The atoms qualify as Parmenidean Reals in two ways. First, like the four elements and the homeomeric substances, atoms cannot be generated, destroyed, or qualitatively changed. In addition, they have an added level of compliance with the Parmenidean demands: the atoms themselves are one in kind. All atoms are made out of the same material. Reality, then, really is one and continuous in at least a qualitative sense.
Though the atoms are materially homogenous (as well as being uniformly impenetrable and indivisible), they do have some variable properties. They differ from one another in shape, arrangement, position, size, and motion. It is by the arrangement and rearrangement of atoms of different shapes, sizes, and motions that the observable world comes into being.
The boldest aspect of the atomist theory, is that, in addition to positing the atoms as Parmenidean Reals, it also posits a void, which is identified explicitly with non-being. There is an extremely good reason for this move: the Eleatics argued that (1) being cannot admit of a vacuum (i.e. empty space) and (2) without a vacuum there can be no movement. Leucippus was impressed by both of these arguments and was persuaded of their truth. However, he was equally certain of the truth of the claim that movement does in fact exist, since he saw movement all around him. Reasoning with these three premises (i.e. the two Eleatic conclusions, and his own observation that motion must exist) he concluded that there must actually be a vacuum and that this vacuum must be identified with not-being. Though the vacuum is non-being, it is nonetheless real. The atoms exist in this vacuum or void and move about in it, giving rise to the observable world.
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.
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.
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.
The explanatory weakness of atomistic determinism is probably what lead Aristotle to reject atomism despite the fact that he preferred this view to all other Presocratic theories (and even to Plato's theories in some instances). Aristotle recognized the fact that the atomists had actually removed too much anthropomorphism and that the explanatory power of their theory was thus enfeebled. They took the extra controlling force out of nature, without any real alternative to put in its stead. Aristotle put this controlling force back into the philosophical picture with his notion of final cause, or teleology.