From her work Potency and Act, which she wrote as a thesis in 1931 but which was not published until after her death:
Genera and species prescribe beforehand a framework that abides throughout the individual's entire duration in being and is concretely fulfilled successively by variable [veränderlich] accidents. We should then take substance here (in the sense of "second substance") as an instantiated general species. It becomes a concrete individual by being successively filled, and for the first time actual being accrues to the concrete individual--with its abiding stock as well as its changing stock.
Actuality cannot be due either to the abiding stock, or to the changing stock, or to the form of the individual; for each of these "abstract parts" of the concrete individual requires the others, nor can the individual being bring them about or draw them into itself. Thus all mutable being, all becoming, points to an upholding outside of itself, to something immutable, to absolutely actual being. What becomes must take its origin [Ursprung] from what is immutably and thereby it must be upheld.
[Edith Stein, Potency and Act, Redmond, tr., Gelber and Leuven, eds. ICS Publications (Washington, DC: 2009) p. 70.]