First propounded in explanation and defense of the reality of motion, the doctrine of act and potency was destined for far wider application. It was in due course found to be relevant to the structure and the incident limitation and multiplicity of created being, as well as, in contrast, to the simplicity and infinity and unicity of God. Its utilization in these areas enlarged its scope immeasurably. It may be fairly said that the focal distinctions of matter and form, of substance and accident, yes of essence and existence are but applications, significant ones at that, of the still more pivotal distinction of act and potency. Here, then, it is commonly said, is a cardinal conception, the heart and focus of Thomistic metaphysics.
This appraisal is to the mark; there is scarcely a metaphysical problem to which the doctrine of act and potency does not lend some manner of solution, and exploring its possibilities has proved fruitful indeed. For all that, however, it would be a fatal illusion to imagine that once the essentials of the doctrine with its real distinction have been set forth, the machinery of logical deduction has only to be put in gear and all the central theses of metaphysics are automatically delivered. The distinctions mentioned above (matter and form, etc.) are original as well as comparable or similar to that of act and potency, and bringing them into line with the latter is in every instance a problem of its own, the solution coming not mechanically but through hard-won insight. We mention this, not to depreciate act and potency, but to caution against their oversimplication. Having made the point, we append, by way of recapitulation, the first two Thomistic theses proposed by the Sacred Congregation of Studies, veritable epitomes of the doctrine of act and potency:
Thesis I: Potency and act so divide being that whatever exists is either pure act, or is necessarily composed of potency and act as its first intrinsic principles. (Potentia et actus ita dividunt ens, ut quidquid est vel sit actus gurus, vel ex potentia et actu tamquam primis atque intrinsecis principiis necessario coalescat.)
Thesis II: Act, because it is perfection, is not limited except by potency, which is a capacity for perfection. Consequently, in any order in which act is pure, it cannot but be unlimited and unique; wherever, on the other hand, it is finite and multiple, it enters into real composition with potency. (Actus, utpote perfectio, non limitatur nisi per potentiam, quae est capacitas perfectionis. Proinde in quo ordine actus est gurus, in eodem non nisi illimitatus et unicus existit; ubi vero est finitus ac multiplex, in veram incidit cum potentia compositionem.)21