Matter and Actuality
Aquinas often describes matter as a being in potentiality (ens in potentia)or as a being that is or exists in potentiality (est in potentia). 1 He also often describes matter as a being in potentiality alone, or as a being that exists only in potentiality. For example, at Summa Theologiae, Ia, 76, 1, responsio, he writes:
Form is actuality and matter is a being in potentiality alone (ens in potentia tantum).
And at De Spiritualibus Creaturis, un., 1, ad 8um, he says:
Matter can not properly be said to exist except in potentiality. (Materia non proprie dici quod est, cum non sit nisi in potentia. ) 2
Matter, as a being in potentiality, but not actuality, contrasts with the subject or individual substance, which is a being in actuality. The contrast is clearly drawn in Aquinas' discussion of change at Summa Theologiae, Ia, 45, 2, ad 2um:
It is essential to change that something which is the same be different now from the way it was before. Sometimes it is the same being in actuality (ens actu) which is different now from the way it was before, as in the case of qualitative, quantitative, and locational change; sometimes it is the same being in potentiality alone (ens in potentia tantum), as happens in substantial change, the subject of which is matter.