How can events in part of the Stoic cosmos be contingent, when all events are necessary? Why does Chrysippus hold that the impossible may flow from the possible, or that while it is possible Dion die, ‘this person be dead’ is impossible. This article constructs a naïve model of Stoic modality in which truth evaluations are grounded in spatial location and motion. This is shown to provide a best fit for the Stoic doctrine, generating the six events which Chrysippus groups (...) into the four Stoic modal categories, and explaining why the Stoics espoused doctrines attacked as contradictory by their opponents. (shrink)