POSSIBILITY. — In logical terms, the possible is that which, once posited, does not have an absurd consequence; it is that which can be and not be, to which in the ontological order there corresponds the “contingent.” In this logical sense, the possible constitutes the sphere of the problematic and corresponds to Aristotle’s “topics” (cf. Met., VIII, 3, 1047 a 24 ff.). In real terms, possibility denotes the capacity or potency of beings in relation to their own act, but this is a defined sense with respect to the original one that embraces all being.
Possibility can be external and internal. “External” or relative possibility (possibilitas extrinseca) relates to the existence or actual realization of things and thus rests on the principle of causality: it connotes the active potency or productive energy of a being that is precisely called “cause” with respect to what it can produce (its effects). The essence of God, insofar as He is Creator, is the source of reality for all beings and of causality for each according to its own nature (St. Thomas, De Potentia, q. 1, a. 1, ad 5). “Internal” possibility, or absolute possibility, on the other hand, relates to the proper essence of each thing and indicates the compatibility of the principles of essence, that is, the non-contradictoriness of their synthesis (Sum. Theol., 1ª, q. 25, a. 3). This possibility is also called “objective” possibility and constitutes the proper essence of things, whose ultimate foundation is the divine essence itself insofar as it is diversely imitable by things; given the infinity of the divine essence, even the possibles are infinite in number because God remains inexhaustible with respect to finite reality (St. Thomas, De Verit., q. 3, a. 2; cf. Sum. Theol., 1ª, q. 45, a. 3). Suárez identifies possibility with “potency” in reality and thus denies the real distinction between essence and act of being in creatures.
In contemporary thought, N. Hartmann finds contradictory the sharp separation that Aristotle makes between potency and act, replacing it with a theory of the possible that conceives it not as something that is merely a “condition” (Zustand) of being placed alongside that of real being but as a modal moment contained and presupposed in the real: as the Megarians already said, wrongly criticized by Aristotle (Met., IX, 3, 1046 b 29), “pure possibility” is a non-sense; only what is real is possible (N. Hartmann, Möglichkeit und Wirklichkeit, Berlin 1938, pp. 13 ff.). Then it can be said that “in the real, the possible is equally actual, so that one can also affirm that every real possible is equally necessary and that what is really possible is also really necessary” (op. cit., p. 126). In this perspective that places the possible within the real, there is no passage to an Absolute that would be the principle of the possible and the real, but finite being resolves itself into its internal relations.