Birth, self-nourishment, growth, generation, decay, these are so many processes that all men attribute to living things belonging to the lowest degree of life, which is called vegetative. As previously noted, the particular scope of this degree of life, its object so to speak, is simply the body that is informed by the soul, and nothing more:
vegetativum . . . habet pro object() ipsum corpus vivens per animam.1
On this level we find three principal functions, differing specifically one from another: nutrition, augmentation or growth, and generation.
1. Nutrition
Of all vital phenomena, one of the most common and most regular in occurrence is nutrition. Living beings, it is clear, cannot continue to live without nourishing themselves. It is obvious to everyone that when an animal or plant stops feeding itself, it dies. The most immediate purpose of nutrition, then, is the preservation of the living being. Its necessity, apparently, derives from the organic nature of the living substance. Simple elements require no such activity; they either exist or not. Living beings, on the other hand, cannot maintain a proper balance and harmony of their various parts without the conserving activity of nutrition.
There are additional reasons for the existence of the nutritive function. To mention one, it is a fact of experience that the two other important activities of vegetative life, growth and generation, do not function in a living being unless it is fed. Thus, on the vegetative level of vital activity nutrition holds a place that is altogether basic.
b) Definition of Nutrition. - In the Commentary De Anima St. Thomas defines nutrition as follows: "Properly speaking, that is said to be nourished which receives something into itself for its preservation":
id proprie nutriri dicimus quod in seipso aliquid recipit ad sui conservationem. 2
Some clarifications of this definition may be noted. Nutrition, strictly speaking, consists neither in the absorption of food nor in the chemical change it undergoes in the process of digestion - a process Aristotle attributes to heat, characterizing it as a sort of cooking. Taken formally, nutrition denotes the conversion of food into the substance of that which is nourished; in other words, it consists in the living being assimilating and making its own another substance, with the result that it preserves its own being and is able to exercise its various activities. Such a process, it should be observed, cannot be reduced to a simple bringing together or juxtaposition of material parts; what it presupposes is nothing less than a real transformation of the added substance.
c) Nutrition and the various degrees of life. - It may be of some advantage to compare nutrition proper with certain other activities that resemble it, both in the inorganic kingdom and in the realm of sense and intellect. As already said, the assimilation of food cannot be leveled down to a mere juxtaposition of material elements. But can it be likened to the physical or chemical generation of substances, as when, according to the ancients, fire becomes air, or in modern terms, elements combine ti form compounds? Admittedly, in both cases, that is, in nutrition and the so-called generation of new substances, one substance is corrupted and transformed into another; but the conditions of these two processes are altogether different. In the generation of elements or compounds the principle and term of the transformation are different; fire, as the ancients thought, becomes air, whereas in nutrition the living being itself is both principle and term of the operation. Put philosophically, nutrition is an immanent activity - something not found in the mere generation of physical substances or compounds.
On the level of sensitive and intellectual life, further comparisons can be made. The process of knowledge, for example, bears some resemblance to bodily nutrition. Both sentient and intelligent beings do, in a way, nourish themselves in the realm of sense and intellect. Do we not, in fact, speak of spiritual nourishment, and of the hunger and thirst of truth? But again the differences are unmistakable. The so-called intentional union of knower and known is something utterly unique. Unlike food in nourishing, neither the knower nor the known is destroyed in the act of knowledge, in which they become one. If anything, it is the knower that becomes the known. Again, the capacity for bodily nourishment is strictly limited; but the range of the faculties of knowledge, especially of intellect, is virtually unlimited.
2. Growth
a) Its purpose. - Living things - and this, too, is a fact of experience - do not reach their full development by one stroke; their natural height in particular does not appear at once. Living beings grow and increase in height by degrees, until they reach a maximum point that seems to correspond to their complete development. Growth, therefore, which is in the category of quantitative increase, gives every indication of being a distinct process that requires a special faculty: the vis augmentativa.
b) Definition of growth. - Before defining growth we must be sure that the quantitative increase of living beings is, in fact, a sufficiently distinct operation to require a special faculty. Might it not be the mere natural result of other vegetative functions, say, of nutrition. To judge from appearances one might think so. Certainly, the growth of a living thing depends on its nourishment. Also, it would seem that the generative function, by which a new being is substantially engendered, should likewise bestow on this being its proper quantity. Despite these considerations, St. Thomas was firm in ascribing to growth a specific determination that could not be reduced to anything found in the other vegetative activities. Consequently, he maintains the existence of a distinct faculty to explain the phenomenon in question.
To speak precisely, the proper object of growth is the quantity of the living being. The faculty corresponding to this activity may be defined as the potency that enables a bodily being endowed with life to acquire its full stature or quantity as well as the full development of its powers:
secunda autem per fectior operatio est augmentum quo aliquid proficit in majorem perfectionem, et secundum quantitatem et secundum virtutem.3
Like every vital operation, the process of growth, having both its principle and termination within the living being, is an immanent activity.
c) Growth and the various degrees of life. - Inanimate things are capable of increase by juxtaposition; but, exception made perhaps of crystals and what modern science calls the ultraviruses, they do not admit of genuine growth. Briefly, growth is an activity that is proper to living beings.
Above the vegetative level of life we find certain processes of development or increase that bear comparison with growth. But the comparisons also bring out the differences. The fact is that in the proper sense of the word quantitative growth does not occur beyond the corporeal world. In these other degrees of life there is only increase according to quality. In his treatise on habits St. Thomas has carefully analyzed the very special conditions of this type of increase.' Interesting as it might be, a discussion of this point would take us too far afield. The mere mention must suffice.
3. Generation
a) Its purpose. - In addition to self-nourishment and the attainment of their full development, living beings also have the power to generate, that is, to produce a being of their own Npccific kind. Aristotelian physics, it is true, spoke of generation with respect to the elements, such as fire and water. But it is clear that in living beings this operation is quite different, with properties all its own. The purpose of generation, also, is sufficiently obvious; moreover, one can consider it from two different aspects.
First, with respect to the individual and the whole of its activities, generation is a term and perfection: a term in reference to the other operations of vegetative life, nutrition and growth, which prepare the way for generation; a perfection inasmuch as to generate means to transmit one's being, to give oneself, thus realizing, in a manner, what is implied in the expression "act of the perfect," actus perfecti.
Secondly, from the standpoint of living beings collectively, generation is meant for a higher purpose, the continuation of the species. From this aspect, the perfect is the species, which endures, and the imperfect is the individual, which cannot live forever. As if to remedy this defect and survive by proxy, the individual imparts its nature to others that will continue its survival. These two purposes and points of view, it need hardly be said, are not exclusive one of the other, but complementary.
b) Definition of generation. - In the Summa St. Thomas defines the generation of living things as follows: "Generation . . . signifies the origin of a living being from a conjoined living principle . . . by way of similitude . . . in the same specific nature":
generatio signified originem alicuius viventis a principio vivente coniuncto . . . secundum rationem similitudinis . . in natura eiusdem speciei.5
In this formulation, which has become classical, the words "origin of a living being" designate what is common to all generation. The words "from a conjoined living principle" tell the specific difference of generation in living beings. The last two qualifications, "by way of similitude" and "in the same specific nature," exclude from the definition such excrescences as the hair, as well as such things as the various secretions of the body. The process by which these things are produced is not a true generation because they do not arise from and result in a nature of the same specific kind.
c) Generation in other levels of being. - As remarked earlier in the realm of beings below vegetative life we find a sort generation, namely, of one element or substance from another. But this process differs from generation proper, most of all because the activity involved is purely transitive.
Likewise on the plane of intellectual or spiritual being, generation in the strict sense does not occur, at least not among created spirits. The "verbum mentis" or concept, in which intellectual knowledge expresses itself, is not of the same nature as the principle from which it proceeds. There is, however, one exception in point; it is found in God, who, as faith teaches, truly begets the second Person of the Blessed Trinity. By contrast with human generation, the manner of this begetting is of such transcendent character as to exclude absolutely all imperfection on the part of God. How to go about elucidating this mystery is a matter for theology.6
4. Concluding Remarks on Vegetative Life
From what has been said it is clear that vegetative life as conceived in Aristotelian philosophy embraces a number of well- Wined and co-ordinated activities involving a certain degree of immateriality and, correlatively, of immanence. Among the three principal functions distinguishable on this level, there exists a priority of order. Nutrition is the basic function, presupposed by the other two. Growth completes nutrition, and the further end of both is generation, in which vegetative life in one respect reaches its culmination.
Much could be said by way of appraisal and criticism of this doctrine of vegetative life, which reveals both keen insight and discrimination. Admittedly, in the light of the great progress made by the various sciences dealing with life, many of its details would need to be reworked. But there is no doubt that the basic insights on which it rests, retain their essential truth and value.