The Scale of Life and of Living Bodies

a) Differences Among Living Bodies — b) The Degrees or Grades of Life

a) Differences Among Living Bodies

All the living things in this bodily world may be grouped into three general classes: those that have growth, those that grow and move and have feeling, and those that grow and move and feel and reason. In other words, the three fundamental classes of living bodies, recognized by everyone, are plants, animals, and human beings.

There are many minor classifications, of course, in the classes of plants and animals. The botanist will present endless litanies of groups and sub-groups. The zoölogist and the biologist will offer general classes or phyla, and carry each phylum down through classes, orders, families, genera, species, races, and varieties. But the botanist recognizes, throughout his lists, one form of life, namely, the vegetal or plant form. And the biologist finds in all the varied objects of his study the sentient or animal form of life. The psychologist must discern, in the major object of his attention, the specifically human form of life, which is intellectual or rational.

Now, are these three forms of life essentially different, one from another, or is their difference merely accidental? All living bodies are at one in the fact that they are alive; all have life in the sense of a capacity for self-perfective immanent activity; all manifest self-movement. In a word, all have life, and life is a single essence, with a single definition. Still, a single essence may have forms which are essentially different. We have just seen that there are essentially different kinds of bodies (viz., living bodies and lifeless bodies), although all are equally bodies, and a body is one definite essence or reality, with a single definition. The fact is, of course, that some bodies are merely bodies and nothing else, while other bodies are bodies plus another essence called life. In the same way, plants, animals, and human beings all have life; but animals possess all that plants have, plus something else, and human beings have all that plants and animals have, plus something else. And the “something else” in each case is something essentially different and superior. We assert that the difference which marks off animal life from vegetal life, and the difference which distinguishes human life from the life of plants and animals, is, in each case, an essential difference. The assertion is not difficult to prove, and we shall pause upon the point for only one brief paragraph.

We have already seen the axiom, “Function follows essence,” an axiom very often quoted in the old Latin formula, agere sequitur esse. The phrase means that as a thing is so it must act. A reality is constituted by its essence; its essence makes it what it is, and therefore determines the scope and character of its activity or operation. Conversely, the operation or activity of a thing,—that is to say, its proper activity,—is an indicator of the basic constitution or essence of that thing, and indicates the capacity and the limitations of that essence. Hence, if one living body has a proper operation for which another living body shows no capacity whatever, there exists, between these two living bodies, an essential difference. For function follows essence, and the proper function of the first body follows from, and indicates, an essence which is in no wise indicated by the proper operation of the second body. In a word, here are two essences. And this is but another way of saying that the difference between the two bodies in question is an essential difference. Now, the operations of sensing, and of conscious local movement, are proper to animals, and are in no wise attributable to plants; plants are not equipped with organs for sensing; they lack the nervous structure required for conscious activity, and they are regularly rooted and fixed in such a way that self-directed local motion is utterly impossible to them. Yet the essence of plants is definitely and completely constituted without these functions, whereas the essence of animals requires them. Different functions indicate different essences. Therefore, between plant and animal there exists an essential difference. Further, the human operations of understanding and willing are entirely beyond the capacity of brute animals; the animal essence is complete without these operations while the human essence requires them, at least in actu primo, in basic capacity, even if this capacity be unrealized in actual operation. Different proper functions indicate different essences. Therefore, between brute animal and man there exists an essential difference.

We conclude, therefore, that there are, in this bodily world, three essentially different forms of life: plant life (or vegetal life); animal life (or sentient life); and human life (or rational or intellectual life).

b) The Degrees or Grades of Life

Not only are there three essentially different forms of bodily life, and consequently three essentially different kinds of living bodies, but the forms and kinds are also grades or degrees. The word “grade” (and, indirectly, the word “degree”) is derived from the Latin gradus “a step.” And a step is a pace forward or back, up or down; it suggests a connection or relation with another situation. A step is like one level of a stairway, or one rung of a ladder, chiefly in this: that the second level or rung has all the elevation of the first, plus its own elevation; and the third level has all the elevation of the first two, plus its own elevation.

In bodily life, and in living bodies, we discern true degrees or grades. The lowest and least complex level of life is plant life. The next level is animal life. A plant is a bodily thing which takes nourishment, grows, and generates its kind. So is an animal. All that a plant can do, an animal does; but an animal does more. An animal, in addition to plant operations, manifests its own proper activity of sensing, appetizing, and moving about. Therefore, plant life and animal life are not merely two forms of life, but two grades of life. The third and highest level of life in living bodies is human life. Man takes nourishment, grows, and propagates his kind, like a plant; he senses, appetizes, and moves about, like an animal; and, in addition to these operations, he exercises his specifically human operations of understanding and willing. Hence, plant life, animal life, and human life are distinct grades of life. And plants, animals, and human beings are distinct grades of living bodies. In the foregoing section of this Article, we have seen that these grades are essentially distinct or different.

Summary of the Article

This brief Article has taught us that all the myriads of living bodies found in the world may be classified under three general heads,—vegetal beings, sentient beings, and human beings; or, more simply, plants, animals, and men. We have learned that these classes of living bodies manifest, respectively, functions or operations which are different, not merely accidentally, but essentially. And since function follows from, and indicates, essence, we have concluded that plants, animals, and human beings are essentially different kinds of living bodies, or, in other words, that they are endowed with essentially different forms of life. We have learned further that these essentially different forms of life are also essentially different degrees or grades of life.