 He offers an analogy designed to appeal to the Epicureans. In Epicurean physics atoms move downwards in straight lines unless disturbed by collisions with other atoms (collisions that are set in train by atomic swerves). The downward motion of the atoms is without a cause if we mean action exerted on them from outside, but their motion is not unqualifiedly uncaused, for it is their nature to move as they do (De fato 25). The voluntary motions of our mind–our volitions–are like this, not the effect of external causes (though there will be external occasions, or as we might say precipitating causes, for them).