Objection 1. It would seem that creatures do not need to be kept in being by God. For what cannot not-be, does not need to be kept in being; just as that which cannot depart, does not need to be kept from departing. But some creatures by their very nature cannot not-be. Therefore not all creatures need to be kept in being by God. The middle proposition is proved thus. That which is included in the nature of a thing is necessarily in that thing, and its contrary cannot be in it; thus a multiple of two must necessarily be even, and cannot possibly be an odd number. Now form brings being with itself, because everything is actually in being, so far as it has form. But some creatures are subsistent forms, as we have said of the angels (50, 2,5): and thus to be is in them of themselves. The same reasoning applies to those creatures whose matter is in potentiality to one form only, as above explained of heavenly bodies (66, 2). Therefore such creatures as these have in their nature to be necessarily, and cannot not-be; for there can be no potentiality to not-being, either in the form which has being of itself, or in matter existing under a form which it cannot lose, since it is not in potentiality to any other form.
Objection 2. Further, God is more powerful than any created agent. But a created agent, even after ceasing to act, can cause its effect to be preserved in being; thus the house continues to stand after the builder has ceased to build; and water remains hot for some time after the fire has ceased to heat. Much more, therefore, can God cause His creature to be kept in being, after He has ceased to create it.
Objection 3. Further, nothing violent can occur, except there be some active cause thereof. But tendency to not-being is unnatural and violent to any creature, since all creatures naturally desire to be. Therefore no creature can tend to not-being, except through some active cause of corruption. Now there are creatures of such a nature that nothing can cause them to corrupt; such are spiritual substances and heavenly bodies. Therefore such creatures cannot tend to not-being, even if God were to withdraw His action.
Objection 4. Further, if God keeps creatures in being, this is done by some action. Now every action of an agent, if that action be efficacious, produces something in the effect. Therefore the preserving power of God must produce something in the creature. But this is not so; because this action does not give being to the creature, since being is not given to that which already is: nor does it add anything new to the creature; because either God would not keep the creature in being continually, or He would be continually adding something new to the creature; either of which is unreasonable. Therefore creatures are not kept in being by God.
On the contrary, It is written (Hebrews 1:3): "Upholding all things by the word of His power."
I answer that, Both reason and faith bind us to say that creatures are kept in being by God. To make this clear, we must consider that a thing is preserved by another in two ways.
First, indirectly, and accidentally; thus a person is said to preserve anything by removing the cause of its corruption, as a man may be said to preserve a child, whom he guards from falling into the fire. In this way God preserves some things, but not all, for there are some things of such a nature that nothing can corrupt them, so that it is not necessary to keep them from corruption.
Secondly, a thing is said to preserve another 'per se' and directly, namely, when what is preserved depends on the preserver in such a way that it cannot exist without it. In this manner all creatures need to be preserved by God. For the being of every creature depends on God, so that not for a moment could it subsist, but would fall into nothingness were it not kept in being by the operation of the Divine power, as Gregory says (Moral. xvi).
This is made clear as follows: Every effect depends on its cause, so far as it is its cause. But we must observe that an agent may be the cause of the "becoming" of its effect, but not directly of its "being." This may be seen both in artificial and in natural beings: for the builder causes the house in its "becoming," but he is not the direct cause of its "being." For it is clear that the "being" of the house is a result of its form, which consists in the putting together and arrangement of the materials, and results from the natural qualities of certain things. Thus a cook dresses the food by applying the natural activity of fire; thus a builder constructs a house, by making use of cement, stones, and wood which are able to be put together in a certain order and to preserve it. Therefore the "being" of a house depends on the nature of these materials, just as its "becoming" depends on the action of the builder. The same principle applies to natural things. For if an agent is not the cause of a form as such, neither will it be directly the cause of "being" which results from that form; but it will be the cause of the effect, in its "becoming" only.
Now it is clear that of two things in the same species one cannot directly cause the other's form as such, since it would then be the cause of its own form, which is essentially the same as the form of the other; but it can be the cause of this form for as much as it is in matter--in other words, it may be the cause that "this matter" receives "this form." And this is to be the cause of "becoming," as when man begets man, and fire causes fire. Thus whenever a natural effect is such that it has an aptitude to receive from its active cause an impression specifically the same as in that active cause, then the "becoming" of the effect, but not its "being," depends on the agent.
Sometimes, however, the effect has not this aptitude to receive the impression of its cause, in the same way as it exists in the agent: as may be seen clearly in all agents which do not produce an effect of the same species as themselves: thus the heavenly bodies cause the generation of inferior bodies which differ from them in species. Such an agent can be the cause of a form as such, and not merely as existing in this matter, consequently it is not merely the cause of "becoming" but also the cause of "being."
Therefore as the becoming of a thing cannot continue when that action of the agent ceases which causes the "becoming" of the effect: so neither can the "being" of a thing continue after that action of the agent has ceased, which is the cause of the effect not only in "becoming" but also in "being." This is why hot water retains heat after the cessation of the fire's action; while, on the contrary, the air does not continue to be lit up, even for a moment, when the sun ceases to act upon it, because water is a matter susceptive of the fire's heat in the same way as it exists in the fire. Wherefore if it were to be reduced to the perfect form of fire, it would retain that form always; whereas if it has the form of fire imperfectly and inchoately, the heat will remain for a time only, by reason of the imperfect participation of the principle of heat. On the other hand, air is not of such a nature as to receive light in the same way as it exists in the sun, which is the principle of light. Therefore, since it has not root in the air, the light ceases with the action of the sun.
Now every creature may be compared to God, as the air is to the sun which enlightens it. For as the sun possesses light by its nature, and as the air is enlightened by sharing the sun's nature; so God alone is Being in virtue of His own Essence, since His Essence is His existence; whereas every creature has being by participation, so that its essence is not its existence. Therefore, as Augustine says (Gen. ad lit. iv, 12): "If the ruling power of God were withdrawn from His creatures, their nature would at once cease, and all nature would collapse." In the same work (Gen. ad lit. viii, 12) he says: "As the air becomes light by the presence of the sun, so is man enlightened by the presence of God, and in His absence returns at once to darkness."
Reply to Objection 1. "Being" naturally results from the form of a creature, given the influence of the Divine action; just as light results from the diaphanous nature of the air, given the action of the sun. Wherefore the potentiality to not-being in spiritual creatures and heavenly bodies is rather something in God, Who can withdraw His influence, than in the form or matter of those creatures.
Reply to Objection 2. God cannot grant to a creature to be preserved in being after the cessation of the Divine influence: as neither can He make it not to have received its being from Himself. For the creature needs to be preserved by God in so far as the being of an effect depends on the cause of its being. So that there is no comparison with an agent that is not the cause of 'being' but only of "becoming."
Reply to Objection 3. This argument holds in regard to that preservation which consists in the removal of corruption: but all creatures do not need to be preserved thus, as stated above.
Reply to Objection 4. The preservation of things by God is a continuation of that action whereby He gives existence, which action is without either motion or time; so also the preservation of light in the air is by the continual influence of the sun.
Source: Summa Theologica (New Advent)