ἀδύνατον δ' εἴναι τὸ μὲν αἴτιον παρεῖναι, οὖ δέ ἐστιν αἴτιον μὴ ὑπάρχειν.
Whereas the past is unchangeable, Chrysippus argued that some future events that are possible do not occur by necessity from past external factors alone, but might (as Aristotle and Epicurus maintained) depend on us.