Chapter 20

46 This is how the case should be argued, rather than seeking refuge in atoms wandering and swerving from their path. “The atom swerves,” he says. First, why? Atoms already had one kind of force of motion from Democritus, that of impact, which he calls a blow; from you, Epicurus, they had that of gravity and weight. What new cause, then, is there in nature to make the atom swerve? Or do they draw lots among themselves to decide which will swerve and which will not? Why do they swerve by a minimal interval, and not a greater one? Why do they swerve by one minimal interval, but not by two or three?

47 This is wishful thinking, not argument. For you do not say that an atom is struck from outside, displaced, and made to swerve; nor that in the void through which the atom travels there was any cause for it not to travel in a straight line; nor has any change taken place in the atom itself to make it cease to follow the natural motion of its own weight. Thus, although he has offered no cause to bring about that swerve, he still thinks he is saying something when he says what everyone’s mind rejects and repudiates.

48 Indeed, no one seems to me to have done more to establish not only fate but the necessity and compelling force of all things, and to abolish the mind’s voluntary movements, than this man who admits he could not otherwise have withstood fate without taking refuge in these invented swerves. For even if atoms existed—though their existence cannot in any way be established to my satisfaction—those swerves could still never be explained. For if nature’s necessity has bestowed on atoms their movement by gravity, since every weight must move and be carried along when nothing hinders it, then this too must be necessary: that certain atoms—or, if they wish, all atoms—should naturally swerve […]

46 That is the right way to conduct this debate. There is no point seeking support from atoms that stray and turn off course. He tells us, “The atom takes a swerve.” My first question is: why? There was already another force moving atoms. In Democritus' account it was a push—his term is a blow. In your account, Epicurus, it was heaviness and weight. What new natural cause, then, turns an atom aside? Do atoms perhaps draw lots to settle which one swerves and which does not? Why should the distance of the swerve be the least possible, never any larger? Why just one of those smallest units, rather than two or three?

47 That is wishing, not reasoning. Your account does not say that a blow from outside displaces an atom and makes it turn aside. Nor does it supply anything in the void it travels through to explain why its travel should cease to be straight. And nothing has changed inside the atom to stop it keeping to the natural motion its own weight gives it. He has supplied no cause for the swerve at all. Even so, he believes he has made a point, although what he says meets rejection and refusal in everyone's mind.

48 Indeed, to my mind nobody does more than this man to confirm fate and, beyond fate, the compulsion and necessity governing all events. Nobody does more to abolish freely chosen movements of the mind. He admits that he could not oppose fate by any other means: his only escape was to invent these swerves. Let us even grant the existence of atoms, though I cannot possibly be persuaded of it. That would still leave those swerves unexplained forever. Suppose natural necessity assigns atoms their motion by weight. After all, anything heavy must move and travel unless something gets in its way. Then swerving must also happen of necessity, as a natural property of some atoms—or of all atoms, if that is what they want […]