On Youth, Old Age, Life and Death

​Moreover, this is clear both in the case of plants and in that of animals—in plants, to those who study their generation from seeds and their graftings and cuttings. For the generation from seeds occurs in all of them from the middle; for since all seeds are bilobed, they are connected where they grow together, and the middle is of each of the two parts; for from this both the stem and the root of the growing plants shoot forth, and their middle is the principle of their growth. Both in graftings and in cuttings this happens especially around the buds; for the bud is a certain principle of the branch, and at the same time a middle, so that they either remove this or insert into this, in order that either the branch or the roots may come from these, as the principle of the stem and root is from the middle. And in blooded animals, the heart is formed first. This is clear from what we have observed to be visible in those still in the process of formation, where possible. Hence, it is necessary that in bloodless animals too, the analogue of the heart is formed first. That the heart is the principle of the veins has been said before in the treatise on the parts of animals; and also that blood is the ultimate nourishment for blooded animals, from which the parts are formed. It is manifest, then, that the power of the mouth performs one service, and that of the stomach another, with respect to food. But the heart is the most sovereign, and brings about the end. So it is necessary that both the principle of the sensory and that of the nutritive soul be in the heart in blooded animals; for the activities of the other parts concerning food are for the sake of its activity; for the sovereign must relate to that for the sake of which it exists, but not in those things which are for its sake, just as the physician relates to health. Moreover, the sovereign organ of the senses is in this in all blooded animals; for in this it is necessary that the common sensorium of all the senses be. We see two senses clearly extending to this, namely taste and touch, so that it is necessary that the others do so too; for in this it is possible for the motion to be produced for the other sense organs, but these do not extend at all to the upper region. Apart from these, if living is in this part in all of them, it is clear that the sensory principle is also necessary in it; for in so far as it is an animal, in this respect we say it lives, and in so far as it is sensory, in this respect we say its body is an animal. Why some of the senses clearly extend to the heart, while others are in the head (wherefore it seems to some that animals perceive through the brain), the cause of these has been stated separately elsewhere. From what has been observed, then, it is clear from what has been said that both the principle of the sensory soul and that of the growth-promoting and nutritive soul are in this part and in the middle of the three parts of the body.

​And according to reason, because we see nature in all cases making the best of what is possible. And when the ruling principle is in the middle of the essence of each, each of the parts—both that which processes the ultimate nourishment and that which receives it—best performs its own function; for it will be so related to each of them, and the middle position of such a ruler is the ruler’s proper place. Furthermore, that which uses and that which is used must differ. And just as their power differs, so their manner might also differ, just as the flutes and that which moves the flutes, namely the hand. If, therefore, the animal is defined by possessing a sensory soul, for blooded animals it is necessary to have this principle in the heart, and for bloodless animals in the analogous part. And all the parts and the whole body of animals have a certain connate natural heat; wherefore while living they appear warm, but when dying and being deprived of life, the opposite. It is necessary, indeed, that the principle of this heat be in the heart for blooded animals, and in the analogous part for bloodless animals; for all parts process and digest food by natural heat, and most of all the most sovereign part. Wherefore, while life remains when the other parts are cooled, when the heat in this part is destroyed, life is utterly lost, because the principle of heat for all is suspended from here, and the soul is, as it were, rekindled in these parts—in the analogous part for bloodless animals, and in the heart for blooded ones. It is necessary, therefore, that living and the preservation of this heat coexist, and that what is called death is the destruction of this.