Theophrastos on the atomic theory.
172. Theophrastos wrote of Leukippos as follows in the First Book of his Opinions:—
Leukippos of Elea or Miletos (for both accounts are given of him) had associated with Parmenides in philosophy. He did not, however, follow the same path in his explanation of things as Parmenides and Xenophanes did, but, as is believed, the very opposite (R. P. 185). They made the All one, immovable, uncreated, and finite, and did not even permit us to search for what is not; he assumed innumerable and ever-moving elements, namely, the atoms. And he made their forms infinite in number, since there was no reason why they should be of one kind rather than another, and because he saw that there was unceasing becoming and change in things. He held, further, that what is is no more real than what is not, and that both are alike causes of the things that come into being; for he laid down that the substance of the atoms was compact and full, and he called them what is, while they moved in the void which he called what is not, but affirmed to be just as real as what is. R. P. 194.
Leukippos and the Eleatics.
173. It will be observed that Theophrastos, while noting the affiliation of Leukippos to the Eleatic school, points out that his theory is, prima facie,[[935]] just the opposite of that maintained by Parmenides. Some have been led by this to deny the Eleaticism of Leukippos altogether; but this denial is really based on the view that the system of Parmenides was “metaphysical,” coupled with a great reluctance to admit that so scientific a hypothesis as the atomic theory can have had a “metaphysical” origin. It is really due to prejudice, and we must not suppose Theophrastos himself believed the two theories to be so far apart as they seem.[[936]] As this is really the most important point in the history of early Greek philosophy, and as, rightly understood, it furnishes the key to the whole development, it is worth while to transcribe a passage of Aristotle[[937]] which explains the historical connexion in a way that leaves nothing to be desired.
Leukippos and Demokritos have decided about all things practically by the same method and on the same theory, taking as their starting-point what naturally comes first. Some of the ancients had held that the real must necessarily be one and immovable; for, said they, empty space is not real, and motion would be impossible without empty space separated from matter; nor, further, could reality be a many, if there were nothing to separate things. And it makes no difference if any one holds that the All is not continuous, but discrete, with its parts in contact (the Pythagorean view), instead of holding that reality is many, not one, and that there is empty space. For, if it is divisible at every point there is no one, and therefore no many, and the Whole is empty (Zeno); while, if we say it is divisible in one place and not in another, this looks like an arbitrary fiction; for up to what point and for what reason will part of the Whole be in this state and be full, while the rest is discrete? And, on the same grounds, they further say that there can be no motion. In consequence of these reasonings, then, going beyond perception and overlooking it in the belief that we ought to follow the argument, they say that the All is one and immovable (Parmenides), and some of them that it is infinite (Melissos), for any limit would be bounded by empty space. This, then, is the opinion they expressed about the truth, and these are the reasons which led them to do so. Now, so far as arguments go, this conclusion does seem to follow; but, if we appeal to facts, to hold such a view looks like madness. No one who is mad is so far out of his senses that fire and ice appear to him to be one; it is only things that are right, and things that appear right from habit, in which madness makes some people see no difference.
Leukippos, however, thought he had a theory which was in harmony with sense-perception, and did not do away with coming into being and passing away, nor motion, nor the multiplicity of things. He made this concession to experience, while he conceded, on the other hand, to those who invented the One that motion was impossible without the void, that the void was not real, and that nothing of what was real was not real. “For,” said he, “that which is strictly speaking real is an absolute plenum; but the plenum is not one. On the contrary, there are an infinite number of them, and they are invisible owing to the smallness of their bulk. They move in the void (for there is a void); and by their coming together they effect coming into being; by their separation, passing away.”
It is true that in this passage Zeno and Melissos are not named, but the reference to them is unmistakable. The argument of Zeno against the Pythagoreans is clearly given; and Melissos was the only Eleatic who made reality infinite, a point which is distinctly mentioned. We are therefore justified by Aristotle’s words in explaining the genesis of Atomism and its relation to Eleaticism as follows. Zeno had shown that all pluralist systems yet known, and especially Pythagoreanism, were unable to stand before the arguments from infinite divisibility which he adduced. Melissos had used the same argument against Anaxagoras, and had added, by way of reductio ad absurdum, that, if there were many things, each one of them must be such as the Eleatics held the One to be. To this Leukippos answers, “Why not?” He admitted the force of Zeno’s arguments by setting a limit to divisibility, and to each of the atoms which he thus arrived at he ascribed all the predicates of the Eleatic One; for Parmenides had shown that if it is, it must have these predicates somehow. The same view is implied in a passage of Aristotle’s Physics.[[938]] “Some,” we are there told, “surrendered to both arguments, to the first, the argument that all things are one, if the word is is used in one sense only (Parmenides), by affirming the reality of what is not; to the second, that based on dichotomy (Zeno), by introducing indivisible magnitudes.” Finally, it is only by regarding the matter in this way that we can attach any meaning to another statement of Aristotle’s to the effect that Leukippos and Demokritos, as well as the Pythagoreans, virtually make all things out of numbers.[[939]] Leukippos, in fact, gave the Pythagorean monads the character of the Parmenidean One.
Atoms.
174. We must observe that the atom is not mathematically indivisible, for it has magnitude; it is, however, physically indivisible, because, like the One of Parmenides, it contains in it no empty space.[[940]] Each atom has extension, and all the atoms are exactly alike in substance.[[941]] Therefore all differences in things must be accounted for either by the shape of the atoms or by their arrangement. It seems probable that the three ways in which differences arise, namely, shape, position, and arrangement, were already distinguished by Leukippos; for Aristotle mentions his name in connexion with them.[[942]] This explains, too, why the atoms are called “forms” or “figures,” a way of speaking which seems to be of Pythagorean origin.[[943]] That they are also called φύσις[[944]] is quite intelligible if we remember what was said of that word in the Introduction ([§ VII].). The differences in shape, order, and position just referred to account for the “opposites,” the “elements” being regarded rather as aggregates of these (πανσπερμίαι), as by Anaxagoras.[[945]]