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.
More is required for a real singular possibility, but when the further requirements are added, such as a contact between the active and passive factor and the absence of an external hindrance, the potency model suggests that the potency can really be actualized only when it is actualized (Met. IX.5, Phys. VIII.1).
As we've seen, possibilists believe that there are singular possibilities (i.e., singular propositions that are possibly true) about things that don't actually exist, viz., possibilities involving mere possibilia.
His problem was, as mentioned, that the assumptions of his modal conceptions pushed him towards a very similar position with respect to singular possibilities.
This paradigm suggests that actualization is the general criterion of the genuineness of possibilities, but the deterministic implications of this view compelled Aristotle to seek ways of speaking about unrealized singular possibilities.
Say that a strict actualist is an actualist who rejects the idea that there are, or could be, singular possibilities that are directly about things that do not exist.
The model of possibility as potency prima facie allowed Aristotle to speak about all kinds of unrealized singular possibilities by referring to passive or active potencies, but taken separately they represent partial possibilities which do not guarantee that their actualization can take place.
Similarly, haecceitists also believe there are singular possibilities that are in a certain clear sense directly about things that don't exist, viz., possibilities that, were they actual, would entail the exemplification of haecceities that are, in the actual world, unexemplified.
Alexander's Peripatetic theory of alternative prospective possibilities could be called the model of diachronic modalities without simultaneous alternatives: there are transient singular alternative possibilities, but those which will not be realized disappear instead of remaining unrealized.
Aristotle speaks about singular future possibilities which may be realized or remain unrealized and which may or may not cease to be antecedent possibilities.
A few lines later, however, Kant refers to "the dynamical law of causality" (in the singular) and "the possibility grounded thereon of inferring a priori from some given existence (a cause) to another existence (the effect)" (A228/B280).
