Concerning Energy
All the faculties we have already discussed, both those of knowledge and those of life, both the natural and the artificial, are, it is to be noted, called energies. For energy is the natural force and activity of each essence: or again, natural energy is the activity innate in every essence: and so, clearly, things that have the same essence have also the same energy, and things that have different natures have also different energies. For no essence can be devoid of natural energy.
Natural energy again is the force in each essence by which its nature is made manifest. And again: natural energy is the primal, eternally-moving force of the intelligent soul: that is, the eternally-moving word of the soul, which ever springs naturally from it. And yet again: natural energy is the force and activity of each essence which only that which is not lacks.
But actions are also called energies: for instance, speaking, eating, drinking, and such like. The natural affections also are often called energies, for instance, hunger, thirst, and so forth. And yet again, the result of the force is also often called energy.
Things are spoken of in a twofold way as being potential and actual. For we say that the child at the breast is a potential scholar, for he is so equipped that, if taught, he will become a scholar. Further, we speak of a potential and an actual scholar, meaning that the latter is versed in letters, while the former has the power of interpreting letters, but does not put it into actual use: again, when we speak of an actual scholar, we mean that he puts his power into actual use, that is to say, that he really interprets writings.
It is, therefore, to be observed that in the second sense potentiality and actuality go together; for the scholar is in the one case potential, and in the other actual.
The primal and only true energy of nature is the voluntary or rational and independent life which constitutes our humanity. I know not how those who rob the Lord of this can say that He became man.
Energy is drastic activity of nature: and by drastic is meant that which is moved of itself.
Source: Exposition of the Faith (New Advent)