De potentia, Q.3, a.5, corp.

“The second argument is that whenever something is found to be in several things by participation in various degrees, it must be derived by those in which it exists imperfectly from that one in which it exists most perfectly: because where there are positive degrees of a thing so that we ascribe it to this one more and to that one less, this is in reference to one thing to which they approach, one nearer than another: for if each one were of itself competent to have it, there would be no reason why one should have it more than another. Thus fire, which is the extreme of heat, is the cause of heat in all things hot. Now there is one being most perfect and most true: which follows from the fact that there is a mover altogether immovable and absolutely perfect, as philosophers have proved. Consequently all other less perfect beings must needs derive being therefrom. This is the argument of the Philosopher (Metaph. ii, I)."

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Return to Lesson 10: The Least Popular of St. Thomas’ Five Ways