Leucippus and Democritus are considered to be the founders of Atomism. Though Democritus place is later in the chronology, along with Socrates, he is generally considered along with Leucippus.
Leucippus seems to have flourished around 440 B.C. He comes from Miletus and carried on the rationalistic and scientific way of philosophy associated with the city. He was much influenced by Parmenides and Zeno. Epicurus, a later follower of Democritus doubted his existence. There are, however, a number of allusions to him by Aristotle and it seems incredible that these would have occurred if he had been merely a myth.
Democritus was from Abdera in Thrace. He flourished around 420 B.C. He travelled widely in eastern & southern lands in search of knowledge. He seems to have visited Egypt and Persia.
Leucippus, if not Democritus, was led to atomism in the attempt to mediate between monism as represented by Parmenides and pluralism as represented by Empedocles. Their theory is remarkably akin to modern physics. They believed that everything consists of atoms and atoms are physically invisible and indestructible. The atoms have always been in motion and there are infinite numbers of atoms. The space between two atoms is empty. Atoms have different shape and size. Whether Atomist considered atoms as weightless is debated. But there is considerable reason to think that weight was not an original property of atoms of Leucippus and Democritus.
It seems more probable that, on their view, atoms were originally moving at random. Democritus said that there was neither up nor down in the infinite void and compared the movement of atom in the soul to that of motes in sunbeam where there is no beam. As a result of collisions the collection of atoms comes to form Vortices.
Contrary to popular perception in antiquity, Atomists were strict deterministic, who believed that everything happens in accordance with the natural laws. Democritus explicitly denied that anything can happen by chance.
Aristotle and others reproached Leucippus and Democritus for not accounting for original motion of atoms (i.e. how the atoms got into motion), but in science causation must start from something, and wherever it starts no cause can be assigned for initial datum. The world may be attributed to a creator, but even the creator himself is unaccounted for.
The atomists sought to explain the world without introducing the notion of purpose or final cause. The “final cause” of an occurrence is an event in the future for the sake of which the occurrence takes place.
Taking an example if we ask, “Why are railways built?” the answer would be because people would travel. This is the “final cause” why railways are built. When we ask “Why” concerning an event, we may mean either of two things. We may mean “What purpose did this event serve?” or we may mean “What earlier circumstances caused this event?”
The answer to the former question is teleological explanation, or an explanation by final causes, the answer to the latter question is mechanistic explanation. The mechanistic question leads to scientific knowledge, while the teleological question does not. The atomist asked the mechanistic question and gave mechanistic answers. Their successors, until the renaissance, were more interested in teleological question, and thus led science up a blind alley.
As regards the teleological explanation, it eventually arrives at creator whose purpose is realized in the course of nature. But if a man is so obstinately teleological as to continue to ask what purpose is served by the Creator, it becomes impious. The conception of purpose, therefore, is only applicable within reality, not to reality as whole.
A similar argument applies to mechanistic explanations. One event is caused by another and other by third and so on. But if we ask for a cause of the whole, we are driven again to creator who must himself be uncaused. All causal explanations, there must have an arbitrary beginning. This is why it is no defect in the theory of atomist to have left the original movements of atoms unaccounted for.
Leucippus was concerned to find a way of reconciling the arguments of Parmenides with the obvious fact of motion and change. Leucippus thought he had a theory which harmonized with sense perception and would not abolish either coming to be and passing away or motion and the multiplicity of things. He made this concession on the facts of perception. On the other hand he conceded to the monists that there could be no motion without void. The result is a theory which he states as follow:
“The void is a “not being”, and no part of “What is” is a “not being”; for 'what is' in the strict sense of term is an absolute plenum. This plenum, however, is not one, on the contrary, it is many, infinite in number and invisible owing to the minuteness of the bulk. The many move in the void (for there is a void): and by coming together they produce coming to be, while by separating they produce passing away. Moreover, they act and suffer action whenever they chance to be in contact (for there they are not one) and they generate by being put together and becoming intertwined. For the genuinely one, on the other hand, there could never have come to be a multiplicity, nor from genuinely many a one: that is impossible.”
It will be seen that there was on which everybody so far was agreed, namely there could be no motion in the plenum. In this, all alike were mistaken. There can be cyclic motion in a plenum, provided it has always existed. The idea was that a thing could only move into an empty space, and that in a plenum, there are no empty spaces. It might be contended, perhaps validly, that motion could never begin in a plenum, but it cannot be validly maintained that it could not occur at all.
To the Greeks, however, it seemed that one must acquiesce in the unchanging world of Parmenides, or admit the void. One may put the Parmenidian position in this way: “You may say that there is the void; therefore the void is not nothing; therefore it is not the void.” It cannot be said that Atomists answered this argument; they merely proclaimed that they proposed to ignore it on the ground that motion is a fact of experience, therefore there must be a void, however difficult it may be to conceive.
Democritus worked out his theories in considerable details and some of the working out is interesting. Each atom, he said, was impenetrable and indivisible because it contained no void. When you use a knife to cut an apple, the knife has to find empty places where it can penetrate; if the apple contained no void, it would be infinitely hard and therefore physically indivisible. Each atom is internally unchanging, and in fact a Parmenidian one. The only thing that atoms do is to move and hit each other, and sometimes to combine. They are of all sorts of shapes; fire is composed of small spherical atoms; and so is the soul. Atoms, by collision produce vortices, which generates bodies and ultimately worlds. There are many worlds, some growing, some decaying; some may have no sun or moon, some several. Every world has a beginning and an end.
Life developed out of the primeval slime. There is some fire everywhere in a living body. Thought is a kind of motion and is thus able to cause motion elsewhere. Perception and thought are physical process. Perception is of two sorts, one of the sense and one of the understandings. Perception of the latter sort depend only on the things perceived, while those of the former sort depends also on our senses, and therefore apt to be deceptive. Like Locke, Democritus held that such qualities as warmth, taste, and color are not really in the object, but are due to our sense organs, while such qualities as weight, density and hardness are really in the object.
Democritus was through-going materialist; for him, the soul was composed of atoms and thought was physical process. There was no purpose in universe, there were only atoms governed by mechanical laws. He disbelieved in popular religion and argued against nous of Anaxagoras. In ethics he considered cheerfulness as the goal of life, and regarded moderation and culture as the best means to it. He disliked everything violent and passionate; he disapproved of sex, because he said, it involved the overwhelming of consciousness by pleasure.
