1. Exposition of the Opinion

186. There is an opinion here [Thomas Aquinas, Sent. IV d.12 q.1 a2 q.2] that “in the case of natural actions substantial forms are not the immediate principle of acting, or the proximate active principle of acting, but they act through the medium of active and passive qualities as their proper instruments, as said in On the Soul 2.4.416b28-29, namely that natural heat is that whereby the soul acts. And therefore qualities do not act by their own virtue but in virtue of the substantial form. Hence their act has not only an accidental form but a substantial one as term; and because of this, generation is the term of alteration. Now such sort of qualities receive instrumental virtue from the fact that they are caused by essential principles; hence, just as the same being, the same as to species, remains in accidents by divine power after the substance is removed, so too does the same virtue as before remain in them. And therefore, just as they were able to cause a change for substantial form before, so are they also able now.”

187. And if argument is made against this that “nothing acts beyond its species” they reply that “nothing acts beyond its species by its own virtue; but it can, by the virtue of something else of which it is the instrument, act beyond its species, as a saw acts for the form of a bench” [Thomas, ibid. ad 2].