Philosophers from the first have been interested in determining the nature of reality and how human beings come to know that reality, assuming that they can. Metaphysics is the attempt to present a coherent and comprehensive explanation of reality, and epistemology is the theory of knowledge, of how human beings learn what they learn and how they can know that what they learn is true. Different philosophers have had different explanations for these concepts, as can be seen by an analysis of the concepts offered by the Greeks Leucippus, Protagoras, Plato, and Aristotle, to take them in chronological order.
Leucippus, along with Democritus, was a leader of a group of philosophers who became known as "the atomists" and whose ideas are remarkably modern, in keeping with contemporary scientific thought. They saw the world as composed of material bodies themselves composed of groups of "atoms," meaning something indivisible, something that cannot be divided into a smaller component (Palmer 34). Leucippus explained how the indivisible atoms could come together to form the world we see around us--the atoms move in the void, collide, and interlock to form larger aggregates which we can then see with our senses (Luce 74).
The theory developed by Leucippus and Democritus was not itself formed from nothing but built on what had gone before, notably the works of Empedocles and Anaxagoras. Where these earlier philosophers had determined that there was some force which created and infused reality, Leucippus saw only a mechanistic universe. The atoms came together not by divine force but by collision. There is no detailed explanation of the philosophy developed by Leucippus, for his ideas are known today only through the few surviving fragments. He apparently had no idea of the cause for the primal motion of the atoms, the motion which brought them into collision and formed larger aggregates over time:
Leucippus evidently did not...