Atomic theory and the concept of atoms was well known among the ancient Greeks. The most highly developed theory was that of Democritus of Abdera. Democritus was probably born about 460 b.c. and died about 370 b.c. He is reputed to have written more than 70 books, although almost none of his writing survives. Most of what we know of Democritus comes from the writings of those who came after him.
Democritus argued that all matter consists of tiny, physically invisible particles. The Greek word atomos, in fact, means "indivisible." Democritus taught that an infinite number of atoms exist and that they are in constant motion. The space between atoms, he said, is occupied by a void. Atoms were never created, according to Democritus, but have always existed, just as they are now. They are also eternal; that is, they cannot be destroyed.. Atoms have physical properties that explain the properties of matter, he said. Atoms of water, for example, are round and smooth, permitting them to slide over each other; conversely, atoms of fire have jagged edges.
Atomic theories were popular among some Greeks because they provided a way of explaining one of the great philosophical questions of the time: Is there any permanence in a world that seems to be filled with constant change? If atoms exist, they constitute an enduring constancy in the world. Therefore, the changes we see, are, in a sense, illusions. They are merely the rearrangement of eternal particles (atoms).
Some scholars believe that Democritus' atomic theory was not really his own, but that of his teacher, Leucippus of Miletus. It is believed that Leucippus was born about 490 b.c., but almost nothing is known about his life. In fact, some scholars question whether such a person ever lived. He is also credited with originating the theory of causality, namely, that everything that happens in nature has a cause.
Democritus' atomic theory was disputed by many of his contemporaries, particularly by the followers of Socrates. Plato and Aristotle rejected the notion of a void and the possibility of particles' being able to move by themselves. Given the enormous influence of this school--especially of Aristotle--on later philosophers, Democritus' ideas eventually fell into decline.
His ideas never really disappeared, however. They were popular among the followers of Epicurus and with the Roman poet and philosopher, Lucretius. Lucretius used Democritus' atomic theory as the basis for his great poem, De rerum natura ("On the Nature of Things"). In the early 1880s, scientist John Dalton refined Democritus' ideas and used them as the basis for his atomic theory.
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