SUMMA THEOLOGICA. Thomas Aquinas
in movement. Therefore all natural things partake of life.
Objection 2: Further, plants are said to live, inasmuch as they in themselves a principle of movement of growth and decay. But local movement is naturally more perfect than, and prior to, movement of growth and decay, as the Philosopher shows (Phys. viii, 56,57). Since then, all natural bodies have in themselves some principle of local movement, it seems that all natural bodies live.
Objection 3: Further, amongst natural bodies the elements are the less perfect. Yet life is attributed to them, for we speak of "living waters." Much more, therefore, have other natural bodies life.
On the contrary, Dionysius says (Div. Nom. vi, 1) that "The last echo of life is heard in the plants," whereby it is inferred that their life is life in its lowest degree. But inanimate bodies are inferior to plants. Therefore they have not life.
I answer that, We can gather to what things life belongs, and to what it does not, from such things as manifestly possess life. Now life manifestly belongs to animals, for it said in De Vegetab. i [*De Plantis i, 1] that in animals life is manifest. We must, therefore, distinguish living from lifeless things, by comparing them to that by reason of which animals are said to live: and this it is in which life is manifested first and remains last. We say then that an animal begins to live when it begins to move of itself: and as long as such movement appears in it, so long as it is considered to be alive. When it no longer has any movement of itself, but is only moved by another power, then its life is said to fail, and the animal to be dead. Whereby it is clear that those things are properly called living that move themselves by some kind of movement, whether it be movement properly so called, as the act of an imperfect being, i.e. of a thing in potentiality, is called movement; or movement in a more general sense, as when said of the act of a perfect thing, as understanding and feeling are called movement. Accordingly all things are said to be alive that determine themselves to movement or operation of any kind: whereas those things that cannot by their nature do so, cannot be called living, unless by a similitude.
Reply to Objection 1: These words of the Philosopher may be understood either of the first movement, namely, that of the celestial bodies, or of the movement in its general sense. In either way is movement called the life, as it were, of natural bodies, speaking by a similitude, and not attributing it to them as their property. The movement of the heavens is in the universe of corporeal natures as the movement of the heart, whereby life is preserved, is in animals. Similarly also every natural movement in respect to natural things has a certain similitude to the operations of life. Hence, if the whole corporeal universe were one animal, so that its movement came from an "intrinsic moving force," as some in fact have held, in that case movement would really be the life of all natural bodies.
Reply to Objection 2: To bodies, whether heavy or light, movement does not belong, except in so far as they are displaced from their natural conditions, and are out of their proper place; for when they are in the place that is proper and natural to them, then they are at rest. Plants and other living things move with vital movement, in accordance with the disposition of their nature, but not by approaching thereto, or by receding from it, for in so far as they recede from such movement, so far do they recede from their natural disposition. Heavy and light bodies are moved by an extrinsic force, either generating them and giving them form, or removing obstacles from their way. They do not therefore move themselves, as do living bodies.
Reply to Objection 3: Waters are called living that have a continuous current: for standing waters, that are not connected with a continually flowing source, are called dead, as in cisterns and ponds. This is merely a similitude, inasmuch as the movement they are seen to possess makes them look as if they were alive. Yet this is not life in them in its real sense, since this movement of theirs is not from themselves but from the cause that generates them. The same is the case with the movement of other heavy and light bodies.
Whether life is an operation?
Objection 1: It seems that life is an operation. For nothing is divided except into parts of the same genus. But life is divided by certain operations, as is clear from the Philosopher (De Anima ii, 13), who distinguishes four kinds of life, namely, nourishment, sensation, local movement and understanding. Therefore life is an operation.
Objection 2: Further, the active life is said to be different from the contemplative. But the contemplative is only distinguished from the active by certain operations. Therefore life is an operation.
Objection 3: Further, to know God is an operation. But this is life, as is clear from the words of Jn. 18:3, "Now this is eternal life, that they may know Thee, the only true God." Therefore life is an operation.
On the contrary, The Philosopher says (De Anima ii, 37), "In living things, to live is to be."
I answer that, As is clear from what has been said ( Question [17], Article [3]), our intellect, which takes cognizance of the essence of a thing as its proper object, gains knowledge from sense, of which the proper objects are external accidents. Hence from external appearances we come to the knowledge of the essence of things. And because we name a thing in accordance with our knowledge of it, as is clear from what has already been said ( Question [13], Article [1]), so from external properties names are often imposed to signify essences. Hence such names are sometimes taken strictly to denote the essence itself, the signification of which is their principal object; but sometimes, and less strictly, to denote the properties by reason of which they are imposed. And so we see that the word "body" is used to denote a genus of substances from the fact of their possessing three dimensions: and is sometimes taken to denote the dimensions themselves; in which sense body is said to be a species of quantity. The same must be said of life. The name is given from a certain external appearance, namely, self-movement, yet not precisely to signify this, but rather a substance to which self-movement and the application of itself to any kind of operation, belong naturally. To live, accordingly, is nothing else than to exist in this or that nature; and life signifies this, though in the abstract, just as the word "running" denotes "to run" in the abstract.
Hence "living" is not an accidental but an essential predicate. Sometimes, however, life is used less properly for the operations from which its name is taken, and thus the Philosopher says (Ethic. ix, 9) that to live is principally to sense or to understand.
Reply to Objection 1: The Philosopher here takes "to live" to mean an operation of life. Or it would be better to say that sensation and intelligence and the like, are sometimes taken for the operations, sometimes for the existence itself of the operator. For he says (Ethic. ix, 9) that to live is to sense or to understand---in other words, to have a nature capable of sensation or understanding. Thus, then, he distinguishes life by the four operations mentioned. For in this lower world there are four kinds of living things. It is the nature of some to be capable of nothing more than taking nourishment, and, as a consequence, of growing and generating. Others are able, in addition, to sense, as we see in the case of shellfish and other animals without movement. Others have the further power of moving from place to place, as perfect animals, such as quadrupeds, and birds, and so on. Others, as man, have the still higher faculty of understanding.
Reply to Objection 2: By vital operations are meant those whose principles are within the operator, and in virtue of which the operator produces such operations of itself. It happens that there exist in men not merely such natural principles of certain operations as are their natural powers, but something over and above these, such as habits inclining them like a second nature to particular kinds of operations, so that the operations become sources of pleasure. Thus, as by a similitude, any kind of work in which a man takes delight, so that his bent is towards it, his time spent in it, and his whole life ordered with a view to it, is said to be the life of that man. Hence some are said to lead to life of self-indulgence, others a