The doctrine of indifference as it appears in later writers certainly tends, as Prantl points out, towards Nominalism, inasmuch as it gives up the substantiality of the universals.
The question then arises, what sort of substance can be predicate; and in the Categories Aristotle gave an answer, which would have been impossible, if he had not, under Plato's influence, accepted both the unity and the substantiality of the universal.
Matter, as an abstract, unperceived substance or cause, is shown to be impossible, an unreal conception; true substance is affirmed to be conscious spirit, true causality the free activity of such a spirit, while physical substantiality and causality are held to be merely arbitrary, though constant, relations among phenomena connected subjectively by suggestion or association, objectively in the Universal Mind.
The necessity and universality of the judgments of causality and substantiality are taken for granted; and there is no investigation of the place held by these notions in the mental constitution.
He was held to this belief in the substantiality of bodies by his Christianity, by the influence of Aristotle, of scholasticism and of Cartesianism, as well as by his own mechanics.
He has a special relation to Fichte in developing the Kantian activity of consciousness into will and substituting activity for substantiality as the essence of soul, as well as in breaking down the antithesis between phenomena and things in themselves.
Substantiality is impossible without causality, and to this as its true correlative we now turn.