Democritus of Abdera
Life and Work
The Atomists is the last Presocratic school of thought. The most important figure is Democritus (c. 460-370 BC), the philosopher who actually developed the atomic theory. He was a pupil of Leucippus (fl. 450-420 BC) who probably initiated the theory. Both Leucippus and Democritus flourished in the city of Abdera. Democritus visited Athens, but he was disappointed for not finding support in his theories. Another important figure is Metrodorus of Chios.
The Atomic Theory
The Atomic Theory is based on the following principles:
(1) Matter consists of separate, partless, solid, eternal, immutable, invisible and intangible unit-particles which are physically and theoretically indivisible atoms (the ‘uncuttable’);
(2) Atoms differ in shape (A from B), position (Z from N) and order (AN from NA) but not in quality;
(3) Empty space or void is necessary for their movement;
(4) Perceptible change and plurality are the result of the transfer of momentum by the moving atoms and such transfer occurs only by contact and not by distinct action.
Atoms and Void
The theory of indivisible atoms should be regarded as a direct reply to Eleatic monism and in particular to Zeno’s argument of infinite divisibility. Contra the Eleatic absolute denial of non-being, the Atomists state that non-being exists as emptiness: ‘what-is’ (to den) is the plenum of atoms, while ‘what-is-not’ (to meden) is the emptiness of void (kenon). Emptiness can explain natural phenomena and physical plurality; what-is-not is in existence spatially as the fundamental prerequisite of physical motion.
Motion and Necessity
Physical motion is the result of reason (logos) and necessity (anagke) and not of divine justice or moral law (dyke). Generation is an arbitrary motion from one state of atomic conglomeration to another through void. A structure of infinite uncuttable and invisible atoms lies behind the world of everyday experience, and consequently perceptible qualities are merely by convention. Reality consists only of atoms and void.
 Leucippus of Elea or Miletus (both accounts are current) had associated with Parmenides in philosophy, but in his view of reality he did not follow the same path as Parmenides and Xenophanes but rather, it seems, the opposite path. For while they regarded the whole as one, motionless, uncreated, and limited, and forbade even the search for what is not, he posited innumerable elements in perpetual motion—namely the atoms—and held that the number of their shapes was infinite, on the ground that there was no reason why any atom should be of one shape rather than another; for he observed too that coming-into-being and change are incessant in the world. Further he held that not-being exists as well as being, and the two are equally the causes of things coming-into-being. The nature of atoms he supposed to be compact and full; that, he said, was being, and it moved in the void, which he called not-being and held to exist no less than being. In the same way his associate, Democritus of Abdera, posited as principles the full and the void.
 Apollodorus in the Chronicles says that Epicurus was instructed by Nausiphanes and Praxiphanes; but Epicurus himself denies this, saying in the letter to Eurylochus that he instructed himself. He and Hemarchus both maintain that there never was a philosopher Leucippus, who some (including Apollodorus the Epicurean) say was the teacher of Democritus.
 Leucippus postulated atoms and void, and in this Democritus resembled him, though in other respects he was more productive.
 Democritus ... met Leucippus and, according to some, Anaxagoras also, whose junior he was by forty years.... As he himself says in the Little World-system, he was a young man in the old age of Anaxagoras, being forty years younger.
 Demetrius in his Homonyms and Antisthenes in his Successions say that he [Democritus] traveled to Egypt to visit the priests and learn geometry, and that he went also to Persia to visit the Chaldaeans, and to the Red Sea. Some say that he associated with the "naked philosophers" in India; also that he went to Ethiopia.
 Leucippus thought he had arguments which would assert what is consistent sense-perception and not do away with coming into being or perishing or motion, or the plurality of existents. He agrees with the appearances to this extent, but he concedes, to those who maintain the One [the Eleatics], that there would be no motion without void, and says that the void is non-existent, and that no part of what is is nonexistent—for what is in the strict sense is wholly and fully being. But such being, he says, is not one; there is an infinite number, and they are invisible because of the smallness of the particles. They move in the void (for there is void), and when they come together they cause coming to be, and when they separate they cause perishing.
 They [Leucippus, Democritus, and Epicurus] said that the first principles were infinite in number, and thought they were indivisible atoms and impassible owing to their compactness, and without any void in them; divisibility comes about because of the void in compound bodies.
 To this extent they differed, that one [Epicurus] supposed that all atoms were very small, and on that account imperceptible; the other, Democritus, that there are some atoms that are very large.
 Democritus holds the same view as Leucippus about the elements, full and void. . . he spoke as if the things that are were in constant motion in the void; and there are innumerable worlds which differ in size. In some worlds there is no sun and moon, in others they are larger than in our world, and in others more numerous. The intervals between the worlds are unequal; in some parts there are more worlds, in others fewer; some are increasing, some at their height, some decreasing; in some parts they are arising, in others failing. They are destroyed by collision, one with another. There are some worlds devoid of living creatures or plants or any moisture.
 Everything happens according to necessity; for the cause of the coming-into-being of all things is the whirl, which he calls necessity.
 As they [the atoms] move, they collide and become entangled in such away as to cling in close contact to one another, but not so as to form one substance of them in reality of any kind whatever; for it is very simple-minded to suppose that two or more could ever become one. The reason he gives for atoms staying together for a while is the intertwining and mutual hold of the primary bodies; for some of them are angular, some hooked, some concave, some convex, and indeed with countless other differences; so he thinks they cling to each other and stay together until such time as some stronger necessity comes from the surrounding and shakes and scatters them apart.
 Democritus says that the spherical is the most mobile of shapes; and such is mind and fire.
 Democritus and the majority of natural philosophers who discuss perception are guilty of a great absurdity; for they represent all perception as being by touch.
 Leucippus, Democritus, and Epicurus say that perception and thought arise when images enter from outside; neither occurs to anybody without an image impinging.
 Democntus explains sight by the visual image, which he describes in a peculiar way; the visual image does not arise directly in the pupil, but the air between the eye and the object of sight is contracted and stamped by the object seen and the seer; for from everything there is always a sort of effluence proceeding. So this air, which is solid and variously colored, appears in the eye, which is moist (?); the eye does not admit the dense part, but the moist passes through.
 We know nothing about anything really, but opinion is for all individuals an inflowing (?of the atoms).
 It will be obvious that it is impossible to understand how in reality each thing is.
 Sweet exists by convention, bitter by convention, color by convention; atoms and void (alone) exist in reality.... We know nothing accurately in reality, but (only) as it changes according to the bodily condition, and the constitution of those things that flow upon (the body) and impinge upon it.
 It has often been demonstrated that we do not grasp how each thing is or is not.
 There are two sorts of knowledge, one genuine, one bastard (or "obscure"). To the latter belong all the following: sight, hearing, smell, taste, touch. The real is separated from this. When the bastard can do no more—neither see more minutely, nor hear, nor smell, nor taste, nor perceive by touch—and a finer investigation is needed, then the genuine comes in as having a tool for distinguishing more finely.
 Naught exists just as much as Aught.