Parenthetically, we now know that atoms do not occasionally swerve, they always move unpredictably whenever they are in close contact with other atoms. Everything in the material universe is made of atoms in unstoppable perpetual motion. Deterministic paths are only the case for very large objects, where the statistical laws of atomic physics average to become nearly certain dynamical laws for billiard balls and planets, and where we have .

So Epicurus' intuition of a fundamental randomness was correct. Just as Democritus' intuition of atoms in a void was confirmed by modern physics, so Epicurus' swerve (the "clinamen") has been confirmed by quantum physics.

Democritus had replaced the gods as explanations of phenomena with his deterministic laws of nature in order to give humans more control over their fate and thus moral responsibility. For similar reasons, Epicurus added an element of chance to provide more control and moral responsibility than physical determinism could provide. He said, in his Letter to Menoeceus, 134,

It is better to follow the myth about the gods than to be a slave of the "fate" of the physicists: for the former suggests a hope of forgiveness, in return for honor, but the latter has an ineluctable necessity.

It is critically important to note that there is very little new of importance in Epicurus, except the specific mechanism of the atomic swerve, that was not already there in Aristotle's treatment of moral responsibility in the Nichomachean Ethics and the Metaphysics.

Aristotle had already argued against the necessity of the atomists. Aristotle also argued against their causal determinism. He did not care for their atheistic dismissal of the gods. But he had unequivocally endorsed chance, itself an atheistic idea flying in the face of the gods' foreknowledge, as the specific means of breaking the causal chain of determinism and necessity.

The difference between Aristotle and Epicurus is then very slight as concerns the problem of free will, except that Aristotle never accepts the existence of any problem. For him, that our voluntary actions are "up to us" is transparent and obvious. The determinism and necessity of the atomists is simply another impractical ideal, as inapplicable to the real world as the transcendental Ideas of his master Plato.

We know Epicurus' work largely from the Roman Lucretius and his friend Cicero. Lucretius describes Epicurus' idea of breaking the causal chain of determinism in De Rerum Natura.

One further point in this matter I desire you to understand: that while the first bodies are being carried downwards by their own weight in a straight line through the void, at times quite uncertain and uncertain places, they swerve a little from their course, just so much as you might call a change of motion. For if they were not apt to incline, all would fall downwards like raindrops through the profound void, no collision would take place and no blow would be caused amongst the first-beginnings: thus nature would never have produced anything.

But if by chance anyone believes it to be possible that heavier elements, being carried more quickly straight through the void, fall from above or, the lighter, and so deal blows which can produce generative motions, he is astray and departs far from true reasoning. For whatever things fall through water and through fine air, these must speed their fall in accordance with their weights, because the body of water and the thin nature of air cannot delay each thing equally, but yield sooner overcome by the heavier ; but contrariwise empty void cannot offer any support to anything anywhere or at any time, but it must give way continually, as its nature demands : therefore they must all be carried with equal speed, although not of equal weight, through the unresisting void. So the heavier bodies will never be able to fall from above on the lighter, nor deal blows of themselves so as to produce the various motions by which nature carries on her processes. Therefore again and again I say, the bodies must incline a little; and not more than the least possible, or we shall seem to assume oblique movements, and thus be refuted by the facts. For this we see to be manifest and plain, that weights, as far as in them lies, cannot travel obliquely, when they drop straight from above, as far as one can perceive; but who is there who can perceive that they never swerve ever so little from the straight undeviating course?

Again, if all motion is always one long chain, and new motion arises out of the old in order invariable, and if the first-beginnings do not make by swerving a beginning of motion such as to break the decrees of fate, that cause may not follow cause from infinity, whence comes this free will in living creatures all over the earth, whence I say is this will wrested from the fates by which we proceed whither pleasure leads each, swerving also our motions not at fixed times and fixed places, but just where our mind has taken us? For undoubtedly it is his own will in each that begins these things, and from the will movements go rippling through the limbs.
(De Rerum Natura), book 2, lines 216-262, Loeb Classical Library, 181)

Note that the Loeb translators have made the common error of substituting "free will" for the first part of Lucretius' description of free will.

The original text says "whence comes this freedom (libera)," not "whence comes this free will." Epicurus and Lucretius need the swerve only to break the causal chains at some point earlier than our willed actions, so that our will can proceed "whither pleasure leads us" and "just where our mind takes us."

Neither Epicurus nor Lucretius likely assumed we could hold our will morally responsible for actions that are purely random, not involving our desires ("whither pleasure") or beliefs ("our mind").

Nevertheless, both thinkers have been misinterpreted and criticized, starting in antiquity with the Stoics, notably Chrysippus, and later the Academic Skeptic Cicero, who in his De Fato lampoons "free will" as based on randomness.

This model of libertarian free will has been attacked for centuries as irrational and unintelligible. This is the randomness objection, the second horn in the dilemma of the standard argument against free will.