The concept of the atom as the smallest, indivisible entity is one of the oldest ideas of science. It has its origin in a philosophical problem the Greeks tried to solve. Heraclitus believed that the basic nature of all things is change. His famous paradox is that one cannot step into the same river twice because the water has moved on between the steps. Parmenides, however, disagreed with Heraclitus and stated that reality is unchangeable and that change is a mere illusion. His pupil Zeno illustrated this by arguing that an arrow cannot be moving because at any one instance it occupies a particular spot.
Democritus (and probably Leucippus, thought to be the father of atomic theory, but of whom little is known) tried to reconcile these ideas. Democritus argued that it is impossible to keep dividing entities into smaller and smaller parts. Change is part of reality, but takes place within the philosophical framework of Parmenides. Change for Democritus was the local motions of parts that in themselves were unchangeable and invisible: the atoms. Atoms themselves must have a place to move. To Democritus atoms comprise all being but do not fill up all space; everything else is void -- what we today would call empty space or the vacuum.
Democritus tried to explain the different forms in which matter exists, and some of his ideas seem quite modern, although coming from a perspective that does not really resemble today's ideas about atoms. For example, although each atom is too small to be observed, material objects appear because we can observe large groups of atoms assembled in a single place. Democritus thought that substances differ from each other because of the shapes, positions, and grouping of the constituent atoms. In solid bodies the atoms stick together. In fluids -- liquids and gases -- atoms do not stick together, they rebound from one another in random directions. Denser bodies are made up of larger, but still indivisible, atoms. In his theory, there is no limit to the size of an atom; he believed that atoms as big as a world could exist somewhere.
Atomism had few followers before the 17th century. Eublides attacked the theory with the paradox of the heap -- there is no such thing as a heap of sand because one grain is not a heap and if you do not have a heap of sand, adding another grain will not produce a heap. The analogy is with the creation of observables by combining unobservables. Aristotle and Plato rejected the theories of Democritus, with Aristotle unable to accept the void.
