“In the case of matter-form union, specification of the kind of being enjoyed by the composite essence, human being or canine being, for instance, is determined by the act principle within the essence, that is, by the substantial form.
“But in the composition of essence and esse within any finite entity, the specification or determination of the kind of being comes not from the side of the act principle – the actus essendi – but from the side of the potency principle, that is, from the essence.
“This is not surprising, of course, since the essence principle itself either is or at least includes a substantial form. While the form is an act principle within the line of essence, in the line of esse that same form, either in itself in the case of a separate substance or together with its matter in the case of a composite entity, is in potency with respect to its act of being.” (1)
(1)↑ John F. Wippel, The Metaphysical Thought of Thomas Aquinas (Washington, D.C.: The Catholic University of America Press, 2000), 104-105.