LeucippusLeucippus and his student Democritus replaced theological and supernatural explanations of phenomena with natural materialist explanations. Leucippus claimed their were no uncaused events.
"Nothing occurs at random, but everything for a reason and by necessity." (Leucippus, Fragment 569 - from Fr. 2 Actius I, 25, 4) οὐδὲν χρῆμα μάτην γίνεται, ἀλλὰ πάντα ἐκ λόγου τε καὶ ὑπ’ ἀνάγκηςThe materialists and determinists assumed the world was completely made of matter, which they postulated to consist of just a few types of invisible particles that could be combined to make all of the visible objects, their properties, and their behaviors. The fundamental elements of their time - earth, water, air, and fire - were in turn simply compounds of sub-elementary particles they called atoms (indivisibles) in a void or vacuum between the atoms. Parmenides had denied the possibility of the void with the simple logical argument that if nothing was between two bodies, it follows that they are in contact with one another. Plato and Aristotle generally preferred Parmenides' idea of a continuous filled plenum and opposed the atomists' ideas of discrete particulate objects separated by nothing. We do not know how many of Leucippus' ideas were shared by his student, because so little is known of his work. He may have, like Democritus, denied the arbitrariness of phenomena that was implied if they were the free actions of the gods. These two thinkers replaced that explanation with the idea of deterministic laws governing the behavior of the atoms, and as a consequence explaining all phenomena made of atoms, including human beings and their actions. In denying the gods and their freedom, Democritus was no doubt aware of the negative implications for human freedom and moral responsibility. Would causal material explanations reduce all events to mere happenings, with no room for intentions, purposes, and human wills? Moral responsibility was very important to Democritus. It was a large part of his reason for eliminating the gods and the idea of fate. Unfortunately, eliminating the gods was impolitic and Democritus' work was shunned by many philosophers, starting with Socrates and Plato. Nevertheless, the view of atoms and a void working by natural causal laws was such a gain over the traditional view of arbitrary fate and capricious gods, that Democritus simply insisted that determinism provided enough responsibility. In this respect, Democritus seems to anticipate the idea of the semi-compatibilism of determinism and moral responsibilty.
νόμωι χροιή, νόμωι γλυκύ, νόμωι πικρόω, ἑτεῆι δ’ ἄτομα καὶ κενόν (Diels Kranz, fragment B125)