Leucippus of Miletus was a contemporary of Melissos. He is said to have "heard" Zeno and may well have visited Elea, the headquarters of the "school" of Parmenides. By inventing the atomic theory he gave what seems much the most satisfactory answer to the question first posed by his fellow-countryman Thales, at least if that question is taken to be a "scientific" one; and, in his own peculiar way, he succeeded in reconciling the views both of the Ionians and of the Eleatics. His theory was developed by Democritus (born about 460 B.C.) and was later expanded into a system that was both philosophical and ethical by Epicurus. At this point I shall make no comment on the later developments. It is, in fact, impossible to distinguish clearly what parts of the development of the theory should be attributed to Leucippus and what to Democritus. Leucippus, however, must be credited with having taken the first and decisive step. He was bold enough to assert that "Nothing" "exists," and was thus able to offer an explanation of motion. He accepted Zeno's arguments against infinite divisibility and asserted the existence of ultimate particles, or "atoms" (a word which simply means "indivisibles"), each one of which had, as Melissos had suggested, the characteristics of the Parmenidean "One". Everything, he declared, was made up of atoms and the void (the void being "nothing") in different arrangements. The atoms themselves are invisible, though they are not all of the same size. Yet, though they have magnitude, they cannot be divided. The reason for this is that they, like the "One" of Parmenides, contain no empty space. The practical applications of this theory will be discussed later, when we come to consider Epicurus. Here it will be sufficient merely to notice Leucippus' revolutionary new use of the verb "to be". His essential principle that there can be a void — that there can be what is, in a sense, nothing — would have seemed an intolerable paradox to earlier thinkers, who were inclined to identify existence with corporeal existence. This bold unorthodoxy was an essential factor in his resistance to the logic of Parmenides and Zeno.
Only one fragment of Leucippus survives. It is "Nothing happens at random, but all things for a reason and of necessity." This is certainly evidence for a belief in determinism; but how far Leucippus himself went with his determinism we do not know. "Necessity" may still have been to him a "force" like the Love and Strife of Empedocles. Possibly the full determinism by which everything takes place simply because of the properties inherent from eternity in the atoms and in the void was the later development of Democritus. Leucippus' importance as a logician is well shown, however, by a passage from Theophrastus and a passage from Aristotle. These I quote from Burnet.
1. Leucippus 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, to all appearance, the very opposite. 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.
2. Leucippus and Democritus 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 made 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.
Leucippus, however, thought he had a theory which was in harmony with sense, and did not do away with coming into being and passing away, nor motion, nor the multiplicity of things. He conceded this 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 is 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."