In his Summa Contra Gentiles Aquinas quotes a passage from Pseudo-Dionysius’s Divine Names revealing the possibilism latent in their shared Platonism:
though anything is good in so far as it is a being… being is a term used absolutely, while good also includes a relation… provided it be ordered to the end, it may be called good because of this relation… It is apparent in this conclusion that good is, in a way, of wider scope than being. For this reason, Dionysius says, in the fourth chapter of On the Divine Names: “the good extends to existent beings and also to non-existent ones.” (SCG 3.20.5)
On the one hand, for Aquinas and Pseudo-Dionysius, there is clearly a sense in which the possible “extends” beyond the merely actual, inasmuch as there are beings, namely “non-existent ones,” that do not exist and yet which are good (i.e., they have an order towards an end, namely the Good). This is Aquinas’s and Pseudo-Dionysius’s “possibilism.” On the other hand, for them the possible is entirely determined and exhausted by the good: to be possible is to be a good, that is, to have an intrinsic order towards the good.