 Aristotle’s conceptual difficulties can be seen from his various attempts to characterize the possibilities based on dispositional properties such as heatable, separable, or countable. Analogous discussions were not unusual in later ancient philosophy. In Philo’s definition of possibility (ca. 300 BCE), the existence of a passive potency was regarded as a sufficient ground for speaking about a singular possibility. The Stoics revised this definition by adding the condition of the absence of external hindrance, thinking that otherwise the alleged possibility could not be realized. According to the deterministic world view of the Stoics, fate as a kind of active potency necessitates everything, but they did not accept the Master Argument of Diodorus Cronus for determinism, which was meant to show that there cannot be possibilities which will not be realized. The number of passive potencies with respect to a definite future instant of time is greater than what will be realized. As long as these possibilities are not prevented from being realized by being unactualized, they in some sense represent open possibilities. Alexander of Aphrodisias was influenced by the Stoic ideas of diachronic modalities, but he thought that it was misleading to speak about unrealized diachronic possibilities if everything is determined. He argued for what he took to be Aristotle’s view, namely that there are undetermined prospective alternatives which remain open options until the moment of time to which they refer. (See Sharples 1983; Bobzien 1993, 1998; Hankinson 1998.) Neither Aristotle nor later ancient thinkers had any considered conception of simultaneous alternatives. They thought that what is, necessarily is when it is, and that the alternative possibilities disappear when the future is fixed. Alexander’s Peripatetic theory of alternative prospective possibilities could be characterized as the model of diachronic modalities without simultaneous alternatives: there are transient singular alternative possibilities, but those which will not be realized vanish instead of remaining unrealized.