Now ‘principle’ [beginning] is placed in the definition of nature as its genus, and not as something absolute, for the name ‘nature’ involves a relation to a principle. For those things are said to be born which are generated after having been joined to a generator, as is clear in plants and animals, thus the principle of generation or motion is called nature. Hence they are to be laughed at who, wishing to correct the definition of Aristotle, tried to define nature by something absolute, saying that nature is a power seated in things or something of this sort.
Ponitur autem in definitione naturae principium, quasi genus, et non aliquid absolutum, quia nomen naturae importat habitudinem principii. Quia enim nasci dicuntur ea quae generantur coniuncta generanti, ut patet in plantis et animalibus, ideo principium generationis vel motus natura nominatur. Unde deridendi sunt qui volentes definitionem Aristotelis corrigere, naturam per aliquid absolutum definire conati sunt, dicentes quod natura est vis insita rebus, vel aliquid huiusmodi. (S. Thomae Aquinatis, In Phys. II)