We must now treat of youth and old age, and of life and death. At the same time it may be necessary to explain the causes and conditions of respiration. For in some animals life and death are conditioned by respiration. We have elsewhere treated more precisely of the soul, and it is clear that its ultimate nature cannot be corporeal, although it has its seat evidently in some part of the body, and in some part, too, that has a higher importance amongst the body’s members. For the present we must dismiss the other parts or powers of the soul (whatever may be the proper term to apply to them). In regard to creatures that are termed animals and have life, in cases where they are endowed with both attributes—I mean with animality and life—it must be that the principle whereby they live, and by virtue of which they are called animal, should be one and the same part. For it is impossible for an animal as animal not to have life. On the other hand, it is not necessary for a thing to be an animal because it has life. For plants live, and yet they have no sensation, and it is in terms of sensation that we distinguish the animal from the non-animal. Numerically they are one and the same part, although in their mode of expression they are manifold and different. For it is not the same thing to be an animal and to have life. Since amongst our sense-organs there is one which we call a kind of ‘common sense,’ where all our actual sensations must come together, this ‘common sense’ must occupy a position midway between what we call the anterior and posterior parts of the body (by anterior is meant that which is situated towards the region of sensation, and by posterior that which is situated in the opposite region). Furthermore, since in all living organisms the body is divided into an upper and lower half (for all animals as well as plants have an upper and lower part), the nutritive principle should evidently occupy a position midway between them. The part which contains the organ for admitting food is called the upper part, and we use the term ‘upper ‘here in reference to the body itself and not in reference to the directions of the surrounding universe; by ‘lower’ we mean the part, whose primary function is to void excrement. In this connection we find a wide difference between plants and animals. In man more than in any of the other animals, owing to his erect attitude, we find the upper part turned towards the upper part of the universe; in the other animals the upper part is turned in a direction midway between the upper and lower parts of the universe; in plants, which are fixed on one spot and draw their nourishment from the earth, this upper part must in every case have a downward direction. For roots in plants and mouths in animals are analogous organs, by means of which in the one case food is derived from the earth, and in the case of animals through themselves.
All perfectly developed animals are analysable into three parts—one for the admission of food, a second for the voiding of excrement, and a third midway between these two. The last of these is called in larger animals the chest, and in the smaller some equivalent term is used; in some animals, however, it is more clearly articulated than in others. Again, such animals as are capable of progressive motion have, in addition to the parts mentioned, other organs adapted to the service of movement and to carrying the entire trunk—as legs, feet, and other organs that have the same function as these. The nutritive principle of the soul is situated in a region central to these three parts, as is evident both from observation and reason. In fact there are many animals which, after one or the other part has been cut off, even the head and the organ for seizing food, continue to live in that part to which the middle is conjoined. This fact may be plainly observed in the case of insects, such as wasps and bees. Also many animals besides insects, when cut in two, can continue to live by the functioning of the nutritive principle. This nutritive part is actually unitary, but potentially multiple, and the natural construction of these animals is the same as that of plants. For plants after they are divided continue to live in segments, and from a single origin one can by section produce several trees. Why it is that some plants cannot continue life after section, while others can be propagated, must be explained elsewhere. In this respect, however, plants and insects as a class are alike. It is necessary that the nutritive principle in those living things that possess it should be actually one, though potentially multiple. And the same holds good of the principle of sensation; for the segments evidently have sensation. In reference to the maintenance of their natural life, plants ([when divided]) are able to survive. Animals are not, for they lack organs for their preservation, wanting in some instances organs for the seizure of food, and in others for receiving it into the body, and in other instances wanting other organs in addition to both of these. Such divisible animals resemble a complex of several creatures grown together. In the most perfectly organized bodies, however, this phenomenon is not found, because their natures have been fashioned into the greatest possible unity. There are always, however, certain dissected members which exhibit slight sensation, because they are still under the influence of a certain psychical affection. For after the entrails have been removed, bodily movements are still continued, as one observes in tortoises after the removal of the heart.
We have further proof of the central situation of the nutritive principle in the case of both plants and animals. In the case of plants we observe their generation from seeds, and we also note the phenomena of grafts and slips. Generation from seeds takes place in every instance from the centre. All seeds are bivalvular, and the point at which their two halves are joined is the point from which generation begins and the middle in reference to the two parts. It is from this point that stem and root shoot forth in growing plants, and the point of origin is also the central point. This phenomenon may be observed in the case of the buds of grafts and slips, for the bud is in a sense the life-principle of the branch and at the same time its centre. It is the bud, therefore, which one removes or into which one inserts a graft, in order to produce branch or root from it, on the theory that the origin of life in branch or root is the centre. In sanguineous animals the first organ in development is the heart. This is plainly seen in those animals whose process of generation admits of observation. In bloodless animals the organ that is analogous to the heart must be the first developed. We have already said in our earlier treatise, On the Parts of Animals, that the heart is the organ from which the veins proceed, and that the blood in sanguineous animals is the ultimate source of nourishment, out of which the members are generated. Although regarding nourishment it is plainly the office of the mouth to perform one certain task, and the office of the stomach to perform another task, yet the heart is the master-organ and sets the end for all the others. Consequently, in sanguineous animals it is in the heart that we must look for the origin of nutrition and sensation. For regarding the preparation of food, the functions of the other organs are subordinate to the function of the heart. It must be true that the master-organ is that which works persistently towards the end and does not stop with that which is subordinate to the end, just as a physician persistently, works towards health. At any rate, the dominating organ of sensation in all sanguineous animals is found in the heart, for the ‘common sense’ which serves all the special senses must be situated there. There are two senses, taste and touch, whose channels lead manifestly to the heart, and what is true of these must be true of the other senses. Movement in the other sense-organs may be transmitted to the heart, but with the upper parts of the body these two senses do not communicate at all. Apart from these considerations, if the life-principle in all animals is situated in the heart, the sensory principle must evidently be found there also. For that by virtue of which we call a thing an animal is the same as that by virtue of which we say that it lives, and the differential mark of sensation is the same as the differential mark of a living body. The reason why certain senses are, as we see, connected with the heart and others with the head (in consequence of which some philosophers regard the brain as the organ of animal sensation), has been given in a separate treatise.
From what we have said, based on observed facts, it is clear that the principle of sensation, as well as that of growth and nutrition, is situated in this organ and in the middle of the three divisions of the body. On the basis of deduction we should say the same thing, because we see that nature, out of existing possibilities, does in every instance the best. Now, if each principle is situated in the central section, the parts (viz. that which finally elaborates the food and that which receives it) would thus perform in the best possible way their several functions. For to each of the parts the central organ will then be so related as is best, and the mid-position in a case such as this is the position that naturally belongs to a ruling principle. Again, one must make a distinction between the user and the instrument used (and as they differ in function, so too, if possible, they should differ in position), as a flute and the hand that plays it are different in function and situation. If, then, to be an animal means to have the power of sensation, this power in the case of sanguineous animals must be found in the heart and in bloodless animals in a corresponding organ. Every member and the entire animal body possess to a certain degree congenital heat. Consequently we see that during life animals are warm, but when dead and deprived of life they are cold. The source of this heat in sanguineous animals must be sought in the heart, and in bloodless animals in the analogous organ. For all the organs (especially the dominating one) prepare and digest their food by means of natural heat. Consequently, all the other parts of the body may become cold and yet life may continue, but when the master-organ becomes cold, life is destroyed entirely, because this is the source of heat for distribution to all other organs, and the soul is as it were suffused with fire in this organ, which in sanguineous animals is the heart, and in bloodless animals an organ analogous to the heart. Life, then, must go hand in hand with the continuance of this heat, and what we call death is its discontinuance.
There are two ways in which, as we see, fire may be extinguished, viz. it may either go out or be put out. In the former case we say the extinction is caused from within, in the other case it is caused by opposing forces; an example of the former is old age; of the latter, external violence. Extinction in both cases, however, is due to the same ultimate cause, viz. the failure of fuel, for when fuel fails and the heart can no longer receive sustenance, extinction of the fire ensues. Cold, by retarding digestion, arrests nourishment. And there are times when it extinguishes itself, as, e.g., when the heat is massed in too great quantity, owing to lack of respiration or lack of cooling. And when the heat accumulates in such mass, it soon causes an exhaustion of fuel, and this process of exhaustion takes place before evaporation has time to develop. Not only, then, is a smaller fire extinguished by a larger one, but also the flame of a lamp is consumed within itself when immersed in a large flame, just as any other combustible would be consumed. The reason for this is that the larger flame uses the fuel contained in the smaller before other fuel can take its place, and the fire continues in constant process of development and in constant flow like a river, but we do not observe this on account of its rapidity. It is clear, then, that if the heat is to continue (and this is necessary if life is to continue), there must be some means of reducing the heat in the chief organ. An illustration of this may be had in what takes place in quenched coals. For if coals are kept closely covered in a common oven, the fire is quickly extinguished. Whereas if one in rapid alternation removes a lid and sets it on again, the coals continue lighted a long time. Covering a fire with ashes also keeps it. For owing to the porosity of the ashes ventilation is not prevented, and the ashes, by admitting the surrounding air, protect the fire against extinction through excess of heat arising in it. However, the explanation of the fact that opposite effects are produced by covering a fire with ashes and with an oven-lid (for the latter extinguishes it and the former keeps it a long time) has been given in our treatise On Problems.
Since every living thing has a soul, and the soul, as we have said, cannot subsist without natural heat, we find that in plants adequate provision has been made for the preservation of natural heat through nutriment and the surrounding air. For food produces refrigeration in organisms when it is first introduced, just as on its entrance it does in man. Whereas fasting creates heat and produces thirst. For air, when it is stagnant, always becomes heated, but when set in motion through the admission of food it is cooled, until the food has undergone digestion. On the other hand, if the surrounding air is excessively cold, owing to the season and the occurrence of heavy frosts, plants are dried up, or if intense heat occurs in summer and the moisture derived from the soil is inadequate for refrigeration, the natural heat is extinguished and destroyed. In such seasons one says that trees are frosted or suffer blight. And that is the reason why people pile certain kinds of stones about the roots or cover them with vessels of water, in order to keep the roots of the plants cool. In the case of animals, since some of them are aquatic and others live in a medium of air, the refrigeration is derived from and through the media in which they live, i.e. in the one case water and in the other air. We must now begin a special inquiry touching the nature and manner of this process.