Again borrowing

borrowing from Fr. Dewan's paper. This is only distantly relevant, but it helps me to think.

If asked to choose a text giving a good first idea of form as here meant, I think of the discussion in the Summa theologiae as to whether death is natural for the human being. I do so, not only to move from the realm of artifacts to that of natural substances, but also to go beyond mere mathematical form. In the text I have in mind Thomas presents the universe in which corruption obviously occurs. The question is whether it is "natural." The individual agents seek to perpetuate their being, but the universal agent[s] regulate this cosmic situation so that equilibrium is maintained. Thus, we read:
. . . we can speak of each corruptible thing in two ways: in one way, in function of universal nature; in another way, in function of the particular nature.
The particular nature is the intrinsic active conserving power of each thing. And [taking "nature"] in that latter way, all corruption and defect is against nature, as is said [by Aristotle] in De caelo, book 2.
But universal nature is the active power in some universal principle of nature, e.g. in one of the celestial bodies, or in some superior substance, according to which [perspective] some people call God "the nature bringing about nature" [natura naturans]. Now, this power intends the good and conservation of the universe, which requires the alternation of generation and corruption in things. And in that perspective corruptions and defects are natural, not according to the inclination of the form, which is the principle of being and perfection, but according to the inclination of the matter, which is proportionately attributed to such a form in accordance with the distribution of the universal agent.
And though every form intends the perpetuation of being to the extent this is possible, nevertheless no form of a corruptible thing can achieve its perpetuity save the rational soul, by the fact that it is not completely subject to corporeal matter as other forms are; indeed, it has its own immaterial operation, as was established in the First Part.
It is such a principle of perpetuity that we mean by "form." This is seen both in the individual seeking self-preservation through nutrition, etc., and in the individual as the embodiment of a specific nature (which is perpetuated).

(..)

The presupposition here is that being is the terminus of generation. And though the examples are taken from the realm of human art, i.e. music and grammar, it is clear enough that for St. Thomas as for Aristotle it is natural generation and natural substance which is the primary focus of metaphysical attention. Thus, the starting-points of metaphysics referred to by St. Thomas in De substantiis separatis are the specifically determinate natural substances: human beings, dogs, trees, minerals.