Objection 1. It seems that to live belongs to all natural things. For the Philosopher says (Phys. viii, 1) that "Movement is like a kind of life possessed by all things existing in nature." But all natural things participate 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.
Source: Summa Theologica (New Advent)