I aim to examine a disagreement between two central Stoic figures, Cleanthes and Chrysippus. Cleanthes, the second head of the Stoa, held that the sage cannot lose virtue once in possession of it, whereas Chrysippus, the third head of the Stoa, believed that virtue can be lost under certain conditions: “And Chrysippus says that virtue can be lost, while Cleanthes says that it cannot be lost (Cleanthes says) it cannot, because (it consists in) secure grasps.” Chrysippus’ position, which I will refer to as the instability thesis, will seem bizarre. I argue Chrysippus was not deviating from previous Stoic thought, but rather, derived his position from Stoic physical theory and was right to do so.