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This article has previously appeared in Philosophes Medievaux Tome XXVI, L’homme et son univers au moyen age, Actes du septième congrès international de philosophie médiévale (30 aoüt - 4 septembre 1982), édités par Christian Wenin. We thank l’Institut Superieur de Philosophie of Lou Vain-LaNeu Ye for granting us permission to republish it in our Thomas International website.




Ralph McInerny



Thomas Aquinas discusses the divine omnipotence in a number of places. The following remarks will be based principally on Summa theologiae, I, q. 25, which is articulated in six subquestions: (1) Whether there is power in God; (2) Whether His power is infinite; (3) Whether He is omnipotent; (4) Whether He can make the past not to have been; (5) Whether He can do what He has not done or do away with what He has done; (6) Whether He can make better what He has already made.

Active, not passive, power is found in God and His power is infinite and unrestricted. If a thing acts insofar as it is in act and God is pure act, active power belongs to Him preeminently. The second article argues:


Active power is found in God insofar as He is in act.

His existence (esse) is unlimited insofar as it is not limited by being received.

So God’s power is infinite.


This is the background against which Thomas asks if God is omnipotent. If God can do anything, what is the meaning of “anything”? The correlative of power (potentia) is the possible and


cum “Deus omnia posse” dicitur, nihil rectius intelligitur quam quod possit omnia possibilia, et ab hoc omnipotens dicitur.


Anything that can possibly be or be done falls within the scope of the divine power.

God’s power relates to possibles absolute, what is possible without qualification.


Dicitur autem aliquid possibile vel impossibile absolute ex habitudine ter minorum: possibile quidem quia praedicatum non repugnat subiecto, ut Socratem credere; impossibile vero absolute, quia praedicatum repugnat subiecto, ut hominem esse asinum.


Whatever in any way whatsoever can be, comes within the range of God’s power and He can make it be. In God’s case, and only in His case, the possible said relatively to an active power and the possible without qualification are the same. God’s power is grounded in (is identical with) His infinite being which is the sum of all perfection. Thus, what is said to be possible with respect to His power is anything whatsoever that can be.

How does Thomas handle the objection: to sin is something, but God cannot sin, therefore there is something God cannot do? “(To sin is to fail to act perfectly, hence the ability to sin is the ability to be defective in action, but that is repugnant to omnipotence”. We have here an instance of the repugnantia terminorum with which Thomas defines impossibility and non-being. A capacity to act is defined in terms of successful, not of defective, action. Being able to fail is not a way of successfully achieving what one sets out to do. Thus, to be able to act without restriction, that is, to be able unqualifiedly to bring action to its intended term, excludes and is incompatible with acting defectively. Thomas thus takes the objection to say: God is not all powerful because He is all powerful. The thing He cannot do is what doing aims to avoid, namely, failure. To do everything well excludes doing something badly, but to say that God cannot act badly does not provide an instance of not acting well of which He is incapable.

The repugnantia terminorum thus seems to be found in the conjunction of some kind of action and our understanding of what God is. If we take the formula “God, who is able to do anything, cannot do X”, Thomas is committed to the view that any value of X will turn out to be a non-thing and thus will not come under the range of “anything”. There are true sentences of the form “God cannot do X”, as for example “God cannot perform an unjust action”, but, as we have seen, an unjust action is a failure in action and to say that God’s action cannot fail is another way of saying that He is omnipotent.

Article 4 of Question 25 provides us with an analysis of this kind. Can God bring it about that what was was not? (a) If God can make the blind see, which looks to be per se impossible (one who cannot see sees), what is per accidens impossible should pose no difficulty for Him. But that a past event not be turns on a contingency: that Socrates ran might not have occurred. So isn’t it only accidentally impossible that that past event should not be? (b) What God could do, He can do. Before Socrates ran, God could bring it about that Socrates not run. Is His power diminished by time such that just because Socrates did in fact run God can no longer bring it about that he did not? (c) If God’s forgiveness restores the charity lost by sin, can He not restore lost virginity too?

God’s power does not encompass the impossible, where the impossible means that which cannot be, that which implies a contradiction. It is a contradiction to say that X exists and that X does not exist. So too to say that X did exist and that X did not exist is a contradiction. God can make what was not to be and what is not to be, but to bring it about that something both was and was not, both is and is not, does not come within the range of His power, because these are impossible. They are nothing and, as Frank Sheed reminded us, nothing is impossible to God.

With respect to (a): the fact that a past event happened contingently and might easily not have happened, does not make it any less impossible for it not to have occurred once it has occurred. As for (b): it is no diminution of God’s power to say that a realized possibility cannot now be unrealized; for a thing to have been and not to have been is impossible. As for (c): God’s mercy bears on the physical as well as the mental aspects of sin; when we are forgiven it is as if we had not sinned insofar as certain consequences of our past acts are removed, but forgiveness does not bring it about that we did not do what we did.

The conception of divine omnipotence Thomas argues for is straightforward. If something can be, God can bring it about; His power extends to any thing or state of affairs which does not involve a contradiction. The contradiction is something in the object of the power (S is and S is not), but sometimes the repugnantia follows on conjoining God and a certain kind of action not in itself impossible. Sinful action is not an impossibility; it is alas all too often instantiated by us. What is impossible is that an all powerful being should sin, since sinning is a defect in action. So too being able to change, to be moved and altered, do not involve contradiction; indeed, the changes which occur are ultimately due to God. Thus, a value of X in “God, who can do all things, can do X” is “effect change”. But “bring it about that He Himself changes” is not a value of X that will deliver a true statement. What we mean by God and what we mean by a being subject to change are incompatible.

In Providence and Evil, Professor Peter Geach has raised objections to Thomas on omnipotence. It should be said at the outset that Geach himself is fundamentally dissatisfied with what he has said about Thomas (pp. 23-4). He takes it that while Thomas seems to wish to maintain that God’s omnipotence enables Him to do anything it is logically possible to do, Thomas really does not wish to hold this understanding of divine omnipotence. There are logically possible things that God cannot do.

Geach begins by distinguishing what he calls four views of omnipotence. It would perhaps be better to see that he proceeds in four stages to forestall certain crude misunderstandings of the claim. Thus he addresses the difficulty that lying is a logically possible feat which God cannot perform by suggesting that this is impossible only if the Christian conception of God is true.

Making a thing which its maker cannot afterward destroy is a logically possible feat. Some humans perform it. If God cannot do this, there is a logically possible feat He cannot perform and the second view of omnipotence — the one Geach takes to be Thomas’s view collapses. He considers the following retort. “God cannot make a thing that He cannot destroy” may be converted to “Anything God can make He can destroy” and that does not look like an objection. Geach distinguishes two ways of understanding the converted proposition: (1) Anything that its maker cannot destroy, God cannot make; (2) God cannot bring about the following feat: to make something that its maker cannot destroy. On the (2) reading, either (2) is false and there is something God cannot do, or it is true that He can make something He cannot destroy and that tells against His omnipotence.

Geach’s third theory, that God can do whatever it is logically possible for God to do, so that the logical impossibility does not reside in X alone but in “God can do X”, is also, he feels, open to challenge. Thomas holds that God cannot bring it about that the past has not been. What was possible is no longer possible, the change having occurred in creatures, not in God. Geach says that if Thomas defends this he has abandoned the position that God can bring about everything describable in a logically consistent way. Geach seems to be saying that Thomas holds that once a possibility, always a possibility. This, like what Geach has to say about potentia absoluta and potentia ordinata, is more a misunderstanding than an objection.

Alluding to the long list of things that an omnipotent God cannot do, given in Summa contra Gentiles, II, 25, Geach says that the mere occurrence of such a list makes him doubt that Thomas really means it when he says God can do anything, even in the highly unnatural sense that “God can do everything that is not excluded under one or other of the following heads”. This seems simply disingenuous. The sentence “There are things which cannot be” or the phrase “impossible things” would seem in Geach’s view to be self-contradictory.

Geach suggests that we replace omnipotence (the claim that God can do anything) with almightiness (the claim that God has power over all things). I think Thomas would want to know a lot more about this positive suggestion since, taking it in one sense, it comes perilously close to saying that God can do whatever God can do. Furthermore, it seems to limit God’s power to the things that actually are or will be, with disastrous consequences, among them that the creatures that are or will be seem the commensurate object of God’s power.

Geach’s objection to the view of omnipotence according to which “God can do anything” and “anything” is whatever does not involve a logical contradiction, is that there are doable things that God cannot do if Christianity is true. In response to Geach, let “anything” stand for “making something the maker cannot unmake or destroy”.


God cannot bring it about that a maker makes something the maker cannot destroy.


This is ambiguous. Does “maker” in the object clause refer to God or to created makers? If it refers exclusively to created makers and we are speaking of God’s primary causality and it is possible for some created maker to make something he cannot subsequently destroy, then the claim is false.

If the claim is taken to mean: God cannot make something God cannot destroy, then it is true, but this is no restriction on the divine omnipotence since a creature which does not depend for its existence on God is a contradiction in terms. Furthermore, the fact that a created maker can make something which that created maker cannot destroy is a sign of the limitation of the created maker, and for God to lack a limitation is no diminution of His excellence or power.

Restrictions of space necessitate a brevity which may unintentionally be unfair to Geach. It is my conviction that nothing Geach has said poses a serious difficulty for the view of divine omnipotence held by Thomas Aquinas. “God can do anything” remains a perfectly defensible statement of divine omnipotence. There is no doable thing that escapes God’s power, because doable thing cannot be instantiated by

(a) self-contradictory descriptions of feats and/or

(b) non-self-contradictory feats the doing of which by God involves a contradiction.