POSSIBILITÀ. – In the logical sense, possible is that which, once posited, has no absurd consequence; it is that which can be and not be, corresponding in the ontological order to the “contingent”; in this logical sense, the possible constitutes the sphere of problematic inquiry and corresponds to Aristotle’s “topical” method (cf. Met., VIII, 3, 1047 a 24 ff.). In the real sense, p. indicates the capacity or potency of beings as related to their act, but this is a defined sense in relation to the original one, which embraces all being.
P. may be external or internal. “External p.” or relative possibility (possibilitas extrinseca) is related to the existence or actual realization of things and therefore rests on the principle of causality: it denotes the active potency or productive energy of a being that is precisely called a “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 (s. Tommaso, De Potentia, q. 1, a. 1, ad 5). “Internal p.”, or possibility in the absolute sense, is instead related to the proper essence of each thing and indicates the compatibility of the principles of the essence, that is, the non-contradictoriness of their synthesis (Sum. Theol., 1°, q. 25, a. 3). This p. is also called “objective p.” and constitutes the proper essence of things, whose ultimate foundation is the divine essence itself insofar as it is variously imitable by things; given the infinity of the divine essence, the possibles are also infinite in number, because God remains inexhaustible in relation to finite reality (s. Tommaso, De Verit., q. 3, a. 2; cf. Sum. Theol., 1°, q. 45, a. 3). Sudre identifies p. in reality with “potency” and therefore 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 the entity, posited alongside that of real being, but as a modal moment contained in and presupposed by the real: as the Megarians, unjustly criticized by Aristotle, had already said (Met., IX, 3, 1046 b 29), the “pure possible” is nonsense; only that which is real is possible (N. Hartmann, Möglichkeit und Wirklichkeit, Berlin 1938, p. 13 ff.). It may then be said that “in the real, the possible is equally actual, so that one may also assert that every real possible is equally necessary, and that what is really possible is also really necessary” (op. cit., p. 126). In this perspective, which locates the possible within the real, there is no transition to an Absolute that is the principle of the possible and the real; rather, finite being is resolved into its internal relations.