 2.80–332. The opening exposition of book 2 descends into the details of atoms’ behaviour and qualities. They are in perpetual motion at enormous speed, since in the void they get no resistance from the medium, and when they collide they can only be deflected, not halted. Their weight gives them an inherent tendency to move downwards, but collisions can divert those motions in other directions. The result is that, when in a cosmic arrangement, atoms build up complex and relatively stable patterns of motion, which at the macroscopic level appear to us as states of rest or relatively gentle motion. Lucretius compares a flock of sheep on a distant hillside, which appears as a stationary white patch, even though close up the constituent sheep prove to be in motion (2.317–22). The most celebrated part of this account, however, is at 2.216–93 (see extended textual discussion in Fowler 2002), where Lucretius maintains that not only to explain how atomic collisions can occur in the first place, but also to account for the evident fact of free will in the animal kingdom, it is necessary to postulate a minimal indeterminacy in the motions of atoms, an unpredictable ‘swerve’ (clinamen) ‘at no fixed place or time’. Otherwise we would all be automata, our motions determined by infinitely extended and unbreakable causal chains. A striking resemblance to the indeterminacy postulated by modern quantum physics—which has also often been invoked in debates about determinism—has helped make this passage the subject of particularly intense debate.  Analogously to various modern philosophical attempts to exploit quantum indeterminacy as a basis for psychological indeterminism, interpreters of Lucretius have long debated what relation he postulates between the swerve and free will. Some have read him as positing at least one atomic swerve in the soul to coincide with (and probably help constitute) every new volition. Others have drawn attention to his remark that the swerve is needed ‘so that cause should not follow cause from infinity’ (2.255) and argued that the theory aims merely to ensure that our present self is not the necessary product of our entire past atomic history. A wide variety of further variants have been proposed.