All the faculties Anast. Sin. in ῞Οδηγ., from Greg. Nyss., p. 44; Clem. Alex. ap. Max., p. 151. 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 The Greek ἐνεργεία being a term with a large connotation is explained as meaning in different cases operation (operatio), action (actio), and act (actus). Nemesius defines actio as operatio rationalis, actus as perfectio potentiæ 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.