1. Distinctive Characteristics of a Living Thing
All men have some knowledge regarding the nature of a living thing and its difference from what is nonliving. This knowledge is common property. What, then, lies at the bottom of these spontaneous notions of the average man? In general the real nature of things does not dwell at the surface, to be seen at a glance. In practice, therefore, the philosopher as well as the common man must judge of their nature by their activities. Accordingly, the notion of life has to be gathered and put together by observing how living things behave, and comparing their behavior with that of nonliving things. This is what Aristotle did, remarking that some natural bodies have life, others not; and by life is meant self-nutrition, growth, and self-decay.1
In his Commentary St. Thomas notes that Aristotle did not propose this observation as a formal definition of life, but merely as an illustration of certain activities that are typical of it. St. Thomas adds that other activities might have been included, at least for the higher forms of life associated with sense and intellect. Thus, not only self-nutrition, growth, and decay, but also the power to sense and to think, and the power to move themselves locally and to procreate are so many operations all men attribute to living things and, conversely, deny of the nonliving.
There is another characteristic that marks off the living being. Unlike what is purely material, a living thing, we say, is an organized being, meaning that it is composed of heterogeneous parts with an orderly arrangement among themselves. A vegetable, for example, has roots, stem or stalk, and leaves and branches. The whole diversified structure gives rise to a harmonious ensemble of functions, operating for the perfection of the whole being. On the other hand, the parts of a mere mineral arc, all of them, homogeneous, at least so far as we can judge from our scale of observation. Actually, however, this second characteristic of life reduces to those of the first group, which are more fundamental.
2. The Formal Definition of Life
Needless to say, we want to know more precisely what it is that separates living things from the nonliving. Even a superficial inspection reveals, among other things, that the former are endowed with a certain interiorness or spontaneity not found anywhere else. For example, it is by its own initiative or power that the animal moves from place to place, and nourishes, and reproduces itself, whereas in the motion, say, of a stone the whole impetus, so far as we can see, comes from the outside. This fact leads us to affirm that the power to move itself by itself is distinctive of the living being, since the nonliving is such as to move only in being moved by another. Of course, as used here, the terms "movement" and "being moved" are taken in their widest acceptation, which includes every kind of change as well as local movement. Such, in general, is the definition of life that has persisted in Aristotelian and Scholastic philosophy. On this point St. Thomas remarks:
Life is essentially that by which a thing is able to move itself, taking the word "movement" in a wide sense, so that even the operation of the intellect can be called "movement." For, those things that can be moved only by an exterior principle are said to be without life.2
A living being, then, is one that can move itself. But what, exactly, does this statement mean? For one thing, it suggests the spontaneity, the inner thrust and impulse, that seems to characterize vital activity. A living being, in other words, has within itself the efficient principle of its activity. This is an accurate observation, but we must not take it to mean more than it does. To say that things having life can move themselves does not imply that the movement of nonliving things in no way proceeds from within, or conversely, that the activity of the living does not depend on exterior conditions. By reason of its form, even a nonliving being may be considered as a kind of principle of activity, but all this means is that the nonliving can transmit, mechanically as it were, the impulse or determination it receives. A living being, however, responds in an original And assimilative manner to the exterior surroundings on which it variously depends; by its own initiative and power it transforms what it receives from without, doing so in a manner that becomes increasingly more individual and personal according to the scale of its activities. At the physiological level this response proper to living things is known as "irritability." At this level, therefore, irritability is said to be the characteristic of life.
Even in this primitive scale, however, the words "to move oneself" have another, and deeper, significance. What they mean is that the living being is the object and term of its own activity, that in some respects living things are ends unto themselves. In contrast, the activities of material bodies appear to have no other purpose than to act upon and transform things exterior to themselves. Living beings, on the other hand, act for their own advantage, seeking both to sustain their own being and to acquire its full development. In some manner and measure their activity remains within them, so that it may be designated as immanent. This quality of immanence, moreover, admits of varying degrees, from the comparatively crude interiority of vegetative life to its highest form in the absolutely perfect possession of self, found in God.
3. The Degrees of Immanence in Vital Activity
Common experience has always agreed - and science has found no conclusive evidence to the contrary - that there are three basic kinds of living beings in nature, namely, vegetables, animals, and humans. Following this general acknowledgement, philosophy recognizes a threefold degree of life: vegetative life in plants, sensitive life in animals, and intellectual life in man, noting, moreover, that the lower degrees of this hierarchy are contained in the higher.
St. Thomas, it is obvious, took special delight in the study of this hierarchy of the degrees of life, leaving us more than one account of it.3 Sometimes he bases the gradation on the degree of immateriality relating to a substantial form and its activities; but generally he prefers to determine the kinds of life according to the degree of immanence found in different operations of life. In the fundamental text on this matter,4 St. Thomas begins with the principle that the more a being is capable of acting by itself, the higher it is in the scale of life. With this principle as a guide he establishes his classification according to the greater or lesser degree of interiority evidenced by the several factors underlying the activity of a living being, these factors being either a principal or instrumental form and the end. Accordingly, he distinguishes three general kinds of living beings in nature:
1) Those beings (plants) in which nature implants both the form and the end of their movement, so that they act as mere instruments of execution in regard to the movement.
2) Those beings (animals) which, while not determining their own end, nevertheless acquire through themselves the forms governing their activities, these forms being the sensible representations that cause them to move themselves.
3) Lastly, those beings (humans) which, being endowed with intellect, are capable both of determining their end and acquiring the form that is the principle of their operations. The doctrine here summarized is set forth at greater length in the following passage from the Summa (Ia, 18, 3), a passage which we have already referred to as the basic text on the matter:
Since a thing is said to live in so far as it operates of itself and not as moved by another, the more perfectly this power is found in anything, the more perfect is the life of that thing. In things that move and are moved a threefold order is found. In the first place the end moves the agent: and the principal agent is that which acts through its form, and sometimes it does so through some instrument that acts by virtue not of its own form, but of the principal agent, and does no more than execute the action. Accordingly there are things that move themselves, not in respect of any form or end naturally inherent in them, but only in respect of the executing of the movement; the form by which they act, and the end of the action being alike determined for them by their nature. Of this kind are plants, which move themselves according to their inherent nature, with regard only to executing the movements of growth and decay.
Other things have self-movement in a higher degree, that is, not only with regard to executing the movement, but even as regards the form, the principle of movement, which form they acquire of themselves. Of this kind are animals, in which the principle of movement is not a naturally implanted form; but one received through sense. Hence the more perfect is their sense, the more perfect is their power of self-movement. . . . Yet although animals sense the form that is the principle of their movement, nevertheless they cannot of themselves propose to themselves the end of their operations, or movements; for this has been implanted in them by nature; and by natural instinct they are moved to any action through the form apprehended by sense.
Hence such animals as move themselves in respect to an end they themselves propose are superior to these. This can only be done by reason and intellect, whose province it is to know the proportion between the cud and the means to that end, and duly coordinate them.
In the last instance there is the further distinction of lower intellects, such as that of man, and the divine intellect. The former are not completely self-determining, being determined at least by the first principles of the mind; whereas the divine intellect, being always in act, is perfectly autonomous and therefore attains the highest possible degree of vital immanence. In the Contra Gentiles St. Thomas takes up the same subject, this time in connection with the doctrine of the Trinitarian processions.5 His starting point here is this: The higher a nature in the scale of being, the more interior will be whatever originates from it. Thus, at the lowest scale of things we find material bodies, from which nothing can issue forth except through the influence of another. In this way fire begets fire, by producing an alteration in another body.
Above material bodies are plants, in which interior emanation of a sort takes place, since it is within the plant that the sap, according to the ancients, is converted into seed. But it is obvious that here there is not perfect interiority, because the emanation in question, the seed, eventually becomes a being entirely separate from the parent plant. Besides, it should be clearly noted that the original principle itself of this emanation comes from without, namely, the nourishment which the plant through its roots receives from the earth.
Superior to plants are animals, in which is found a higher degree of life, having its principle in the sensitive soul. On this level the emanation results in a term that is truly immanent, for the sensory image or form impressed on the senses, proceeds to the imagination from which it is conveyed to and stored in the memory. However, principle and term of the emanation are still separate and distinct, for the sensory powers cannot reflect on themselves.
It is only with intellect, a reflective power, that we encounter the highest degree of life. Yet even here there are gradations, since the interiorness marking the activity of this faculty may be realized more or less perfectly, depending on the intellect in question. At the lowest level is the intellect of man, since man depends on something outside himself for the starting point of his intellectual activity. The intellect of the angel is higher, for the angel knows himself directly through himself, yet by an act of knowledge that is distinct from his substance. It is only in the utterly perfect unity and immanence of God that vital activity reaches its absolute perfection.6
In short, vital activity on the one hand and immanence or interiorness on the other, are correlative terms whose progressively higher manifestation corresponds to the hierarchy of perfection in living beings. Furthermore, the notion of life, being realized in a manner that is proportional to the various degrees of this hierarchy, is essentially an analogical notion. Consequently, the life of a plant, of an animal, of man, and an angel or pure spirit, are not specifically the same; and in man, moreover, in whom several degrees of life exist together, there is only an analogical proportion between the activity of one degree and another. These points should be made clear at the outset, if only to caution against a univocal interpretation of the notions in question.