  Like Philo and Diodorus, Chrysippus distinguished four modalities and considered them modal values of propositions rather than modal operators; they satisfy the same standard requirements of modal logic. Chrysippus’ definitions are (D. L. 7.75): An assertible is possible when it is both capable of being true and not hindered by external things from being true. An assertible is impossible when it is [either] not capable of being true [or is capable of being true, but hindered by external things from being true]. An assertible is necessary when, being true, it either is not capable of being false or is capable of being false, but hindered by external things from being false. An assertible is non-necessary when it is both capable of being false and not hindered by external things [from being false]. Chrysippus’ modal notions differ from Diodorus’ in that they allow for future contingents and from Philo’s in that they go beyond mere conceptual possibility.