Summa contra Gentiles2.68 Chapter LXVIII. How an intellectual substance can be the form of the body

Accordingly from the foregoing arguments we are able to conclude that an intellectual substance can be united to the body as its form. For if an intellectual substance is not united to the body merely as its mover, as Plato stated, nor is in contact with it merely by the phantasms, as Averroes held, but as its form; and if the intellect whereby man understands is not a preparedness in human nature, as Alexander maintained, nor the temperament, as Galen said, nor harmony, according to Empedocles, nor a body, nor the senses or imagination, as the ancients asserted, it follows that the human soul is an intellectual substance united to the body as its form. This can be made evident as follows.

For one thing to be another’s substantial form, two conditions are required. One of them is that the form be the principle of substantial being to the thing of which it is the form: and I speak not of the effective but of the formal principle, whereby a thing is, and is called a being. Hence follows the second condition, namely that the form and matter combine together in one being, which is not the case with the effective principle together with that to which it gives being. This is the being in which a composite substance subsists, which is one in being, and consists of matter and form. Now an intellectual substance, as proved above/ is not hindered by the fact that it is subsistent, from being the formal principle of being to matter, as communicating its being to matter. For it is not unreasonable that the composite and its form itself should subsist in the same being, since the composite exists only by the form, nor does either subsist apart from the other.

It may however be argued that an intellectual substance cannot communicate its being to corporeal matter, so that the intellectual substance and the corporeal matter have together one being: because different genera have different modes of being, and a more noble mode belongs to a more noble substance. This would be said reasonably if this being belonged in the same way to matter as to the intellectual substance. But it is not so. For it belongs to corporeal matter as its recipient and subject raised to something higher, while it belongs to the intellectual substance as its principle, and in accordance with its very nature. Wherefore nothing prevents an intellectual substance from being the human body’s form, which is the human soul.

In this way we are able to perceive the wondrous connection of things. For we always find the lowest in the higher genus touching the highest of the lower genus: thus some of the lowest of the animal kind scarcely surpass the life of plants, such as oysters which are immovable, have only the sense of touch, and are fixed to the earth like plants. Hence Blessed Dionysius says (Div. Nom. vii.) that Divine wisdom has united the ends of higher things with the beginnings of the lower. Accordingly we may consider something supreme in the genus of bodies, namely the human body equably attempered, which touches the lowest of the higher genus, namely the human soul, and this occupies the last degree in the genus of intellectual substances, as may be seen from its mode of understanding. Hence it is that the intellectual soul is said to be on the horizon and confines of things corporeal and incorporeal, inasmuch as it is an incorporeal substance, and yet the form of a body. And a thing is not less one that is composed of an intellectual substance and corporeal matter, than that which results from the form of fire and its matter, but perhaps more so: since the more a form overcomes matter, the more one is that which is made from it and matter.

Now though form and matter have one being, it does not follow that matter always equals the being of the form. In fact, the more noble the form, the more it surpasses matter in its being. This is clear to one who looks into the operations of forms, from the consideration of which we know their natures, since a thing operates according as it is. Consequently a form whose operation surpasses the condition of matter, itself also surpasses matter in the excellence of its being.

For we find certain lowest forms, which are capable of no operation except such as comes within the compass of the qualities which are the dispositions of matter, for instance heat, cold, moisture and dryness, rarity, density, gravity and levity, and the like; such are the elemental forms. Consequently these forms are altogether material, and wholly merged in matter.

Above these we find the forms of mixed bodies; and these, although they do not extend to any operations that cannot be accomplished through the aforesaid qualities, nevertheless sometimes produce those effects by a higher power which they receive from the heavenly bodies, and which is consequent upon their species; thus the loadstone attracts iron.

Again, above these we find certain forms whose operations include some which surpass the power of the aforesaid qualities, although the same qualities assist organically in their operation; such are the souls of plants; and these again are like not only to the powers of heavenly bodies, in surpassing the active and passive qualities, but also to the movers of heavenly bodies, inasmuch as they are the principles of movement in living things, which move themselves.

Above these forms we find other forms like the higher substances, not only in moving, but also in knowing, and thus they are capable of operations to which the aforesaid qualities do not help even organically, and yet these operations are not accomplished save by means of a corporeal organ: such are the souls of dumb animals. For sensation and imagination are not accomplished by heating and cooling, although these are necessary for the due disposition of the organ.

And above all these forms we find a form like the higher substances even as regards the kind of knowledge, which is intelligence: and thus it is capable of an operation which is accomplished without any corporeal organ at all. This is the intellective soul, for intelligence is not effected by a corporeal organ. Consequently it follows that this principle whereby man understands, namely the intellective soul, which surpasses the condition of corporeal matter, is not wholly encompassed by and merged in matter, as are other material forms. This is indicated by its operation, in which corporeal matter has no part. And yet since the human soul’s act of intelligence needs powers, namely imagination and sense which operate through corporeal organs, this by itself shows that the soul is naturally united to the body in order to complete the human species.