The source text of the Fourth Way

STA says twice in the Fourth Way that the argument is taken from Metaphysics II. The specific text is pretty clearly 993b 24. The Greek text is garbled, but STA smooths it out and gives a commentary that concludes to the existence of a separate form who is the cause of existence. For another translation, see here, paragraphs 292-5.

Whatever is called supreme among other things is so in virtue of something being caused in those others that is predicated of them univocally, in the way that fire is the cause of heat in the elements. And so since heat is said univocally of fire and the thing that is composed of elements, it follows that fire is the hottest thing.

[Aristotle] mentions univocation because sometimes an effect is not similar to its cause in a way that makes it of the same species, due to the excellence of the cause. The sun, for example, is a cause of heat in lower things, but the inferior things cannot receive the effects of the sun or of other celestial bodies so as to be one species with them, since they share in matter. Because of this we do not say that the sun is the hottest thing in the way fire is, but that the sun is something more than even what is hottest.

Truth, however, is not limited to a species but relates to all that is, and because the cause of truth is one with its effect in both name and logos, it follows that what is a cause to things derived from it, so far as they are true, is the supreme truth.

Aristotle later concludes that the principles of things that always exist, sc. the celestial bodies, are necessarily supremely true. He gives two reasons: (1) they are not “sometimes true and sometimes not” and because of this they transcend what is generable and corruptible in truth, that sometimes exist and sometimes do not. (2) Nothing is a cause of the celestial bodies else unless it is a cause of their being. And because if this something transcends even the celestial bodies both in being and truth, since even if these are incorruptible they nevertheless have a cause of being moved and even of their being, as the Philosopher explicitly says.

This has to be the case since it is necessary that all things that are composite or exist by taking part in something else reduce, as to their causes, to things which exist by definition (quae sunt per essentiam). All corporeal things are actual beings only so far as they exist by taking part in forms. So there must be some separate substance which exists by definition which is the source of corporeal substance.


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