Objection 1. It seems that life is not properly attributed to God. For things are said to live inasmuch as they move themselves, as previously stated (2). But movement does not belong to God. Neither therefore does life.
Objection 2. Further, in all living things we must needs suppose some principle of life. Hence it is said by the Philosopher (De Anima ii, 4) that "the soul is the cause and principle of the living body." But God has no principle. Therefore life cannot be attributed to Him.
Objection 3. Further, the principle of life in the living things that exist among us is the vegetative soul. But this exists only in corporeal things. Therefore life cannot be attributed to incorporeal things.
On the contrary, It is said (Psalm 83:3): "My heart and my flesh have rejoiced in the living God."
I answer that, Life is in the highest degree properly in God. In proof of which it must be considered that 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 to 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. Such as have only the sense of touch, as shellfish, move only with the motion of expansion and contraction; and thus their movement hardly exceeds that of plants. Whereas such as have the sensitive power in perfection, so as to recognize not only connection and touch, but also objects apart from themselves, can move themselves to a distance by progressive movement. Yet although animals of the latter kind receive through sense the form that is the principle of their movement, nevertheless they cannot of themselves propose to themselves the end of their operation, or movement; 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 end and the means to that end, and duly coordinate them. Hence a more perfect degree of life is that of intelligible beings; for their power of self-movement is more perfect. This is shown by the fact that in one and the same man the intellectual faculty moves the sensitive powers; and these by their command move the organs of movement. Thus in the arts we see that the art of using a ship, i.e. the art of navigation, rules the art of ship-designing; and this in its turn rules the art that is only concerned with preparing the material for the ship.
But although our intellect moves itself to some things, yet others are supplied by nature, as are first principles, which it cannot doubt; and the last end, which it cannot but will. Hence, although with respect to some things it moves itself, yet with regard to other things it must be moved by another. Wherefore that being whose act of understanding is its very nature, and which, in what it naturally possesses, is not determined by another, must have life in the most perfect degree. Such is God; and hence in Him principally is life. From this the Philosopher concludes (Metaph. xii, 51), after showing God to be intelligent, that God has life most perfect and eternal, since His intellect is most perfect and always in act.
Reply to Objection 1. As stated in Metaph. ix, 16, action is twofold. Actions of one kind pass out to external matter, as to heat or to cut; whilst actions of the other kind remain in the agent, as to understand, to sense and to will. The difference between them is this, that the former action is the perfection not of the agent that moves, but of the thing moved; whereas the latter action is the perfection of the agent. Hence, because movement is an act of the thing in movement, the latter action, in so far as it is the act of the operator, is called its movement, by this similitude, that as movement is an act of the thing moved, so an act of this kind is the act of the agent, although movement is an act of the imperfect, that is, of what is in potentiality; while this kind of act is an act of the perfect, that is to say, of what is in act as stated in De Anima iii, 28. In the sense, therefore, in which understanding is movement, that which understands itself is said to move itself. It is in this sense that Plato also taught that God moves Himself; not in the sense in which movement is an act of the imperfect.
Reply to Objection 2. As God is His own very existence and understanding, so is He His own life; and therefore He so lives that He has not principle of life.
Reply to Objection 3. Life in this lower world is bestowed on a corruptible nature, that needs generation to preserve the species, and nourishment to preserve the individual. For this reason life is not found here below apart from a vegetative soul: but this does not hold good with incorruptible natures.
Source: Summa Theologica (New Advent)