linguistic:translation, structural:scholar_edition
Physics

Epicurus writes in his Letter to Herodotus (not the historian) that " nothing ever arises from the nonexistent ", indicating that all events therefore have causes, regardless of whether those causes are known or unknown. Similarly, he also writes that nothing ever passes away into nothingness, because, "if an object that passes from our view were completely annihilated, everything in the world would have perished, since that into which things were dissipated would be nonexistent." He therefore states: "The totality of things was always just as it is at present and will always remain the same because there is nothing into which it can change, inasmuch as there is nothing outside the totality that could intrude and effect change." Like Democritus before him, Epicurus taught that all matter is entirely made of extremely tiny particles called " atoms " (Greek: ἄτομος; atomos, meaning "indivisible"). For Epicurus and his followers, the existence of atoms was a matter of empirical observation; Epicurus's devoted follower, the Roman poet Lucretius, cites the gradual wearing down of rings from being worn, statues from being kissed, stones from being dripped on by water, and roads from being walked on in On the Nature of Things as evidence for the existence of atoms as tiny, imperceptible particles.

Also like Democritus, Epicurus was a materialist who taught that the only things that exist are atoms and void. Void occurs in any place where there are no atoms. Epicurus and his followers believed that atoms and void are both infinite and that the universe is therefore boundless. In On the Nature of Things, Lucretius argues this point using the example of a man throwing a javelin at the theoretical boundary of a finite universe. He states that the javelin must either go past the edge of the universe, in which case it is not really a boundary, or it must be blocked by something and prevented from continuing its path, but, if that happens, then the object blocking it must be outside the confines of the universe. As a result of this belief that the universe and the number of atoms in it are infinite, Epicurus and the Epicureans believed that there must also be infinitely many worlds within the universe.

Epicurus taught that the motion of atoms is constant, eternal, and without beginning or end. He held that there are two kinds of motion: the motion of atoms and the motion of visible objects. Both kinds of motion are real and not illusory. Democritus had described atoms as not only eternally moving, but also eternally flying through space, colliding, coalescing, and separating from each other as necessary. In a rare departure from Democritus's physics, Epicurus posited the idea of atomic "swerve" (παρέγκλισις parénklisis; Latin: clinamen), one of his best-known original ideas. According to this idea, atoms, as they are travelling through space, may deviate slightly from the course they would ordinarily be expected to follow. Epicurus's reason for introducing this doctrine was because he wanted to preserve the concepts of free will and ethical responsibility while still maintaining the deterministic physical model of atomism. Lucretius describes it, saying, "It is this slight deviation of primal bodies, at indeterminate times and places, which keeps the mind as such from experiencing an inner compulsion in doing everything it does and from being forced to endure and suffer like a captive in chains."

Epicurus was first to assert human freedom as a result of the fundamental indeterminism in the motion of atoms. This has led some philosophers to think that, for Epicurus, free will was caused directly by chance. In his On the Nature of Things, Lucretius appears to suggest this in the best-known passage on Epicurus' position. In his Letter to Menoeceus, however, Epicurus follows Aristotle and clearly identifies three possible causes: "some things happen of necessity, others by chance, others through our own agency." Aristotle said some things "depend on us" (eph'hemin). Epicurus agreed, and said it is to these last things that praise and blame naturally attach. For Epicurus, the "swerve" of the atoms simply defeated determinism to leave room for autonomous agency.