Whether a variety of organic parts is necessary to the animal.
"Certain parts of bodies, etc." One asks whether an animal needs a variety of organic parts.
1. It seems not, because the more perfect the form, the more operations it can perform and with fewer intermediaries. For the sun, owing to its perfection, can act by means of its own light in more operations than some inferior thing. But the form of man is more perfect than the form of an element or a stone; therefore, it can act in more operations and with fewer intermediaries and, as a consequence, requires less diversity in its matter.
2. Likewise, if an animal needed a variety of parts, then this would be the case either for the sake of its essence or for the sake of its operation. But it is not for the sake of its essence, because it is simpler than the essence of other mixed bodies and freer from matter. Nor is it for the sake of its operations, because a plant has more parts but nevertheless has fewer operations.
3. Likewise, the Philosopher asks in On Heaven and Earth," why some spheres have one star and some have several, and some have several motions and some have fewer. And he says that the sphere that is more perfect has fewer motions, and this is why the starry heaven, which is the cause of being and permanence, moves with a single motion or by fewer motions than some other, inferior sphere. Therefore, since these inferior things imitate the superior as far as they are able, then, to the extent that something is more perfect, to that extent it will have fewer motions. And as a consequence the human, since he is the most perfect animal, will have less need for diverse parts since the diverse parts exist only on account of diverse motions.
On the contrary. As the Philosopher says in the third book of On the Soul, "Nature does nothing in vain, nor is it lacking things necessary to it." But nature provides each animal diverse organic parts. Therefore, these are necessary to it.
One must say that an animal needs a variety of organs. The reason for this is that the more perfect the form, the more operations it can perform. But matter is, to a certain extent, an impediment to operations; for a form bound to matter is perforce constrained or limited by it. Thus a form separated from matter, like intelligence, can be almost infinite with respect to a material form. Therefore, a form bound to a uniform matter has a uniform activity. Thus, each part of fire is fire and each part warms like the whole fire. Since, then, the soul is a more perfect form than the form of an element or a stone, it is the source of several operations. But it cannot perform many operations when it is united to matter, unless its matter is diversified, because it can only perform a uniform operation through a uniform matter. And this is why, if the whole animal body were just like the eye, it would not hear nor would it smell, and if its power were proportionate it would see by means of the whole body. And this is why it is necessary that the body, which is the animal's matter, be diversified into parts, so that diverse operations may be performed by means of the diverse parts, because if the body were undiversified [ unigeneum] in its parts then the animal could perform only undiversified actions.
Nevertheless one must understand through the arguments that there is a difference between these inferior things and the superior ones. For among superior ones it is the case that the more perfect something is, the better it attains its end with fewer motions. Thus the supreme sphere attains its end by a single motion, and the inferior one by several. And similarly, when the (better, worse) as well as, perhaps, distance (higher, lower) first cause understands itself it understands all things, whereas inferior substances understand in a less comely manner. But it is different among these inferior things, because the more perfect something is the better it attains its end through several motions. And the reason for this is that these inferior things are joined to matter, and the form is the principle of acting or moving and operating. Therefore, just as the more distant something is from matter the more it shares in the perfection of the form, so too the more distant it is from matter the more it has of [form's] operation. And this is why, when it comes to material things, the human, since he is the most perfect animal, attains his end through several operations. Thus, the order is reversed between material and immaterial things. And the whole reason is that in material things perfection is attained in accordance with the remoteness from matter, but in immaterial ones it is attained in accordance with their nearness [approximation to the simplest principle.
1. On to the arguments. To the first argument one must reply that the more perfect the form the more it extends its operation to several things. Nevertheless it is not necessary that it have several operations. Rather, that it have several operations stems from the perfection of a material thing. Now, an immaterial thing or the first cause can affect more in a single act than a material thing can with several. Thus the proper or characteristic act of the sun is to give light, but through this act it extends itself to a multitude of others.
Thus, one must reply to the form of the argument that a plurality of operations can be observed in two ways, either from the standpoint of the agent's power or from the standpoint of the things affected. In the first way, an animal has several operations; in the second way, the sun does. And this is why an animal requires several parts, with which it operates as if with instruments, just as the sun does, because just as the hand and the foot and the eye are the animal's instruments, so too these inferior things are the sun's instruments, because a human can generate a human only through a human and a cow can generate a cow only through a cow.
2. To the second argument one must say that the animal essence is the root cause of diversity in the parts and its operation is just like a final cause. Now because the animal soul is more perfect with respect to its powers than is the form of inanimate things, and each power requires a different organ, for this reason from the beginning [ originaliter] a diversity of organs is required owing to the essence of the soul itself, just as diversity in the matter is required owing to a diversity of form, and in the end [finaliter] diversity is required owing to the diverse operations which are performed only through diverse organs.
3. To the third argument one must say, just as we said, that the motion of inferior and superior things is different. Nevertheless, if the argument is made about a star, just as already has been done concerning its motion, then one will conclude the opposite, that it is the case among the spheres that a superior sphere has a greater number of stars than does a lower one. Therefore it will be the case among inferior things that a more perfect body will have a greater plurality of operations and powers, and a plurality of powers demands a plurality of organs. This is why it follows that an animal has more need of diverse parts than does something inanimate. And this is true.
-  Throughout this section A. uses superior and inferior, implying quality