Leontius of Byzantium’s treatise Contra Nestorianos et Eutychianos begins with a section in which the author demonstrates how the human nature in Christ can be real without being a second hypostasis. Leontius starts from the ontological model of the Cappadocians but modifies it radically when he complements the two sets of qualities that constitute ‘nature’ and ‘hypostasis’ with an unqualified substrate. Introduction of such a substrate, which the Cappadocians had rejected, ensured the reality of the human nature within the hypostasis of the Word because it served to anchor the set of human qualities, which when seen by themselves were considered to be a mere abstraction. With this new ontological framework Leontius could defend the formula of Chalcedon against its Nestorian and Monophysite detractors and also demonstrate that it did not violate the tenets of Aristotelian philosophy, which in the sixth century was regarded as a true reflection of the order of being.
Cf. Basil of Caesarea, Epistula214.4, ed. Y. Courtonne, Saint Basile, Lettres, vol. 2 (Paris, 1961), pp. 205-206: ἕκαστος . . . ἡµῶν καὶ τῷ κοινῷ τῆς οὐσίας λόγῳ τοῦ εἶναι µετέχει. For the term συνδροµή in this sense, cf. Pamphilus, Quaestio I, ed. Declerck, p. 130, ll. 58-60.
This has already been noted by Daley, ‘ “Richer Union” ’, p. 250and note 59, who concludes that even substantial qualities are accidents and only substance is ‘an existing thing’ but does not explain how he conceives of this substance.