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.

We know Epicurus' work largely from the Roman Lucretius and his friend Cicero. Lucretius describes Epicurus' ideas in De Rerum Natura, 2, lines 216-262.

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.