SUMMA THEOLOGICA. Thomas Aquinas
the soul it is a body, and is organic, and has life potentially. Yet the first act is said to be in potentiality to the second act, which is operation; for such a potentiality "does not reject"---that is, does not exclude---the soul.
Reply to Objection 2: The soul does not move the body by its essence, as the form of the body, but by the motive power, the act of which presupposes the body to be already actualized by the soul: so that the soul by its motive power is the part which moves; and the animate body is the part moved.
Reply to Objection 3: We observe in matter various degrees of perfection, as existence, living, sensing, and understanding. Now what is added is always more perfect. Therefore that form which gives matter only the first degree of perfection is the most imperfect; while that form which gives the first, second, and third degree, and so on, is the most perfect: and yet it inheres to matter immediately.
Reply to Objection 4: Avicenna held that the substantial forms of the elements remain entire in the mixed body; and that the mixture is made by the contrary qualities of the elements being reduced to an average. But this is impossible, because the various forms of the elements must necessarily be in various parts of matter; for the distinction of which we must suppose dimensions, without which matter cannot be divisible. Now matter subject to dimension is not to be found except in a body. But various bodies cannot be in the same place. Whence it follows that elements in the mixed body would be distinct as to situation. And then there would not be a real mixture which is in respect of the whole; but only a mixture apparent to sense, by the juxtaposition of particles.
Averroes maintained that the forms of elements, by reason of their imperfection, are a medium between accidental and substantial forms, and so can be "more" or "less"; and therefore in the mixture they are modified and reduced to an average, so that one form emerges from them. But this is even still more impossible. For the substantial being of each thing consists in something indivisible, and every addition and subtraction varies the species, as in numbers, as stated in Metaph. viii (Did. vii, 3); and consequently it is impossible for any substantial form to receive "more" or "less." Nor is it less impossible for anything to be a medium between substance and accident.
Therefore we must say, in accordance with the Philosopher (De Gener. i, 10), that the forms of the elements remain in the mixed body, not actually but virtually. For the proper qualities of the elements remain, though modified; and in them is the power of the elementary forms. This quality of the mixture is the proper disposition for the substantial form of the mixed body; for instance, the form of a stone, or of any sort of soul.
Whether the intellectual soul is properly united to such a body?
Objection 1: It would seem that the intellectual soul is improperly united to such a body. For matter must be proportionate to the form. But the intellectual soul is incorruptible. Therefore it is not properly united to a corruptible body.
Objection 2: Further, the intellectual soul is a perfectly immaterial form; a proof whereof is its operation in which corporeal matter does not share. But the more subtle is the body, the less has it of matter. Therefore the soul should be united to a most subtle body, to fire, for instance, and not to a mixed body, still less to a terrestrial body.
Objection 3: Further, since the form is the principle of the species, one form cannot produce a variety of species. But the intellectual soul is one form. Therefore, it should not be united to a body which is composed of parts belonging to various species.
Objection 4: Further, what is susceptible of a more perfect form should itself be more perfect. But the intellectual soul is the most perfect of souls. Therefore since the bodies of other animals are naturally provided with a covering, for instance, with hair instead of clothes, and hoofs instead of shoes; and are, moreover, naturally provided with arms, as claws, teeth, and horns; it seems that the intellectual soul should not have been united to a body which is imperfect as being deprived of the above means of protection.
On the contrary, The Philosopher says (De Anima ii, 1), that "the soul is the act of a physical organic body having life potentially."
I answer that, Since the form is not for the matter, but rather the matter for the form, we must gather from the form the reason why the matter is such as it is; and not conversely. Now the intellectual soul, as we have seen above ( Question [55], Article [2]) in the order of nature, holds the lowest place among intellectual substances; inasmuch as it is not naturally gifted with the knowledge of truth, as the angels are; but has to gather knowledge from individual things by way of the senses, as Dionysius says (Div. Nom. vii). But nature never fails in necessary things: therefore the intellectual soul had to be endowed not only with the power of understanding, but also with the power of feeling. Now the action of the senses is not performed without a corporeal instrument. Therefore it behooved the intellectual soul to be united to a body fitted to be a convenient organ of sense.
Now all the other senses are based on the sense of touch. But the organ of touch requires to be a medium between contraries, such as hot and cold, wet and dry, and the like, of which the sense of touch has the perception; thus it is in potentiality with regard to contraries, and is able to perceive them. Therefore the more the organ of touch is reduced to an equable complexion, the more sensitive will be the touch. But the intellectual soul has the power of sense in all its completeness; because what belongs to the inferior nature pre-exists more perfectly in the superior, as Dionysius says (Div. Nom. v). Therefore the body to which the intellectual soul is united should be a mixed body, above others reduced to the most equable complexion. For this reason among animals, man has the best sense of touch. And among men, those who have the best sense of touch have the best intelligence. A sign of which is that we observe "those who are refined in body are well endowed in mind," as stated in De Anima ii, 9.
Reply to Objection 1: Perhaps someone might attempt to answer this by saying that before sin the human body was incorruptible. This answer does not seem sufficient; because before sin the human body was immortal not by nature, but by a gift of Divine grace; otherwise its immortality would not be forfeited through sin, as neither was the immortality of the devil.
Therefore we answer otherwise by observing that in matter two conditions are to be found; one which is chosen in order that the matter be suitable to the form; the other which follows by force of the first disposition. The artisan, for instance, for the form of the saw chooses iron adapted for cutting through hard material; but that the teeth of the saw may become blunt and rusted, follows by force of the matter itself. So the intellectual soul requires a body of equable complexion, which, however, is corruptible by force of its matter. If, however, it be said that God could avoid this, we answer that in the formation of natural things we do not consider what God might do; but what is suitable to the nature of things, as Augustine says (Gen. ad lit. ii, 1). God, however, provided in this case by applying a remedy against death in the gift of grace.
Reply to Objection 2: A body is not necessary to the intellectual soul by reason of its intellectual operation considered as such; but on account of the sensitive power, which requires an organ of equable temperament. Therefore the intellectual soul had to be united to such a body, and not to a simple element, or to a mixed body, in which fire was in excess; because otherwise there could not be an equability of temperament. And this body of an equable temperament has a dignity of its own by reason of its being remote from contraries, thereby resembling in a way a heavenly body.
Reply to Objection 3: The parts of an animal, for instance, the eye, hand, flesh, and bones, and so forth, do not make the species; but the whole does, and therefore, properly speaking, we cannot say that these are of different species, but that they are of various dispositions. This is suitable to the intellectual soul, which, although it be one in its essence, yet on account of its perfection, is manifold in power: and therefore, for its various operations it requires various dispositions in the parts of the body to which it is united. For this reason we observe that there is a greater variety of parts in perfect than in imperfect animals; and