 Alexander turns Chrysippus’ analogy of the cylinder rolling down the slope against the Stoic position. Chrysippus says that, while an initial shove is required to get the cylinder moving, whether and how the cylinder rolls depends on its shape, and likewise, while an initial impression is needed to get a person moving, how they respond to the impression depends on the type of person they are, and thus they are responsible for how they act. But, says Alexander, round things must roll down a slope when shoved; it is not up to them whether or not to roll. Instead, they are compelled to roll by a kind of internal necessity. And if the Stoics are right, vicious people are likewise compelled by their vicious natures, by a kind of internal necessity, to do their evil deeds (Alexander, On Fate ch. 17).