Protomatter

Perhaps the last named, mass-energy, comes closest to converying the Aristotelian idea of protomatter as the basic stuff of the universe. Whatever quarks may be, or leptons and hadrons in their various forms, it seems generally agreed that all are manifestations of mass-energy, the ultimate matrix to which science seems to have come in identifying the material cause of the universe.

Source: wiktionary

The owl's process of assimilation makes it impossible any longer for the mouse's substantial form to animate the mouse's protomatter, so it is replaced in its functioning by the substantial form of the owl. A mouse can become an owl (or part of an owl) preciesely because there is inherent in the mouse a principle of determinability that includes a capability to be owl. That principle, protomatter, is actualized (by the mouse-form) in the mouse, but is potentiality to be actualized in other ways.

Source: wiktionary

Rephrasing Heisenberg's solution, one could say that the explanation of massive, kinetic, and electromagnetic phenomena, all of which may be regarded as real, requires some underlying substrate or ultimate matter or protomatter that itself is real but only in a potential way.

Source: wiktionary

Protomatter is a potential for existence that requires the creative act of a substantial form to bring a particular substance into existence. In other words, although protomatter could exist as any particular substance, substantial form constrains protomatter into a particular substance.

Source: wiktionary