There are two requisites for one thing to be the substantial form of another. One requisite is that the form be the principle of substantial being to that whereof it is the form: I do not mean the effective, but the formal principle, whereby a thing is and is denominated 'being.'* The second requisite is that the form and matter should unite in one 'being'; namely, in that being wherein the substance so composed subsists. There is no such union of the effective principle with that to which it gives being.* A subsistent intelligence, as shown in Chap. LVI, is not hindered by the fact that it is subsistent from communicating its being to matter, and becoming the formal principle of the said matter. There is no difficulty in the identification of the being, in virtue of which the compound subsists, with the form itself of the said compound, since the compound is only through the form, and neither subsist apart.*
It may be objected that a subsistent intelligence cannot communicate its being to a material body in such a way that there shall be one being of the subsistent intelligence and the material body: for things of different kinds have different modes of being, and nobler is the being of the nobler substance. This objection would be in point, if that being were said to belong to that material thing in the same way in which it belongs to that subsistent intelligence. But it is not so: for that being belongs to that material body as to a recipient subject raised to a higher state; while it belongs to that subsistent intelligence as to its principle and by congruence of its own nature.
In this way a wonderful chain of beings is revealed to our study. The lowest member of the higher genus is always found to border close upon the highest member of the lower genus. Thus some of the lowest members of the genus of animals attain to little beyond the life of plants, certain shellfish for instance, which are motionless, have only the sense of touch, and are attached to the ground like plants. Hence Dionysius says: "Divine wisdom has joined the ends of the higher to the beginnings of the lower."* Thus in the genus of bodies we find the human body, composed of elements equally tempered, attaining to the lowest member of the class above it, that is, to the human soul, which holds the lowest rank in the class of subsistent intelligences. Hence the human soul is said to be on the horizon and boundry line between things corporeal and incorporeal, inasmuch as it is an incorporeal substance and at the same time the form of a body.
Above other forms there is found a form, likened to the supramundane substances in point of understanding, and competent to an activity which is accomplished without any bodily organ at all; and this is the intellectual soul: for the act of understanding is not done through any bodily organ. Hence the intellectual soul cannot be totally encompassed by matter, or immersed in it, as other material forms are: this is shown by its intellectual activity, wherein bodily matter has no share. The fact however that the very act of understanding in the human soul needs certain powers that work through bodily organs, namely, phantasy and sense, is a clear proof that the said soul is naturally united to the body to make up the human species.*
2.67 : Against those who maintain that the Potential Intellect is the Phantasy
2.69 : Solution of the Arguments alleged to show that a Subsistent Intelligence cannot be united with a Body as the Form of that Body