Abstract: Central passages: Boethius On Interpretation II 234–5; Diogenes Laertius 7.75; Plutarch, On Stoic Self-contradictions 1055d–f; Alexander of Aphrodisias On Fate 176.14–23.In the Stoic debate over fate and determinism, the modalities (in particular necessity and possibility) played a role in a number of different contexts. Chrysippus rejected Diodorus Cronus’ modal theory because of its built-in necessitarian consequences. He developed his own set of modal notions, which, in themselves, do not lead to necessitarianism and which secure a necessary condition for free action—namely, the existence of occurrences that are both possible and non-necessary (i.e. are contingent). Chrysippus’ modalities are not epistemic. Some critics of the Stoics developed arguments to show that there is a conflict between Chrysippus’ modal notions and the Stoic theory of fate. Later Stoics replied to such objections by giving an epistemic interpretation of Chrysippus’ modal concepts. Critics of the Stoics objected that fate, qua Necessity, renders all events necessary. However, this objection is not justified within Chrysippus’ philosophy.

Keywords: Chrysippus, Diodorus Cronus, epistemic modalities, external force, modal concepts, modality, necessitarianism, necessity, possibility,