Summa contra Gentiles2.25 Chapter XXV. How the almighty is said to be unable to do certain things

From the foregoing we may gather that though God is almighty, He is nevertheless said to be unable to do certain things.

For it was shown above that in God there is active potentiality: while it had already been proved in the First Book that there is no passive potentiality in Him: whereas we are said to be able in respect of either potentiality. Wherefore God is unable to do those things the possibility of which belongs to passive potentiality. What suchlike things are must be the subject of our inquiry.

In the first place, then, active potentiality is directed to action, while passive potentiality is directed to being. Consequently potentiality to being is in those things only which have matter subject to contrariety. Since therefore passive potentiality is not in God, He is unable as regards anything that appertains to His being. Therefore God cannot be a body, and so forth.

Again. The act of this passive potentiality is movement. Wherefore God, to Whom passive potentiality is unbecoming, cannot be changed. It may be further concluded that He cannot be changed in respect of each kind of movement: for instance that He cannot be increased, nor diminished, nor altered, nor generated, nor corrupted.

Moreover. Since to fail is a kind of corruption, it follows that He is unable to fail in anything.

Further. Every failing is in respect of some privation. But the subject of privation is the potentiality of matter. Therefore He can nowise fail.

Again. Since weariness results from defect of power, and forgetfulness from defect of knowledge, it is clear that He can neither be weary nor forget.

Moreover. Nor can He be overcome or suffer violence. For these things happen only to those things that are of a movable nature.

Likewise neither can He repent, nor be angry or sorrowful: since all these denote passion and defect.

Again. Since the object and effect of an active potentiality is something made, and since no potentiality is operative, if the ratio of object be lacking,—thus the sight sees not if the actually visible be lacking:—it follows that God is unable to do whatever is contrary to the ratio of being as being, or of made being as made. What these things are, we must inquire.

In the first place that which destroys the ratio of being is contrary to the ratio of being. Now the ratio of being is destroyed by the opposite of being: as the ratio of man is destroyed by the opposite of man or of his parts. Now the opposite of being is not-being. Consequently God is unable to do this, so as to make the one and same thing to be and not to be at the same time; which is for contradictories to be simultaneous.

Again. Contradiction is included in contraries and privative opposites: for to be white and black is to be white and not white, and to be seeing and blind is to be seeing and not seeing. Hence it amounts to the same that God is unable to make opposites to be simultaneously in the same subject and in the same respect.

Moreover. The removal of an essential principle of a thing implies the removal of the thing itself. If, then, God cannot make a thing at the same time to be and not to be, neither can He make a thing to lack any of its essential principles while the thing itself remains: for instance that a man have no soul.

Further. Since the principles of certain sciences, for instance of logic, geometry, and arithmetic, are taken only from the formal principles of things, on which the essence of those things depends, it follows that God cannot make the contraries of these principles: for instance, that genus be not predicable of species, or that lines drawn from centre to circumference be not equal, or that the three angles of a rectilinear triangle be not equal to two right angles.

Hence it is also evident that God cannot make the past not to have been. Because this also includes a contradiction, since it is equally necessary for a thing to be while it is, and to have been while it was.

There are also some things which are incompatible with the ratio of thing made, as made. These also God cannot do, since whatever God makes, must be something made. Hence it is evident that God cannot make God. For it belongs to the ratio of thing made that its being depends on another cause. And this is contrary to the ratio of that which we call God, as is evident from the foregoing.

For the same reason God cannot make a thing equal to Himself. Because a thing whose being depends not on another, is greater in being and other excellencies than that which depends on another, which belongs to the ratio of a thing made.

Likewise God cannot make a thing to be preserved in being without Himself. For the preservation of a thing in being depends on its cause. Wherefore if the cause be removed, the effect must needs be removed. Consequently, if there could be a thing that is not preserved in being by God, it would not be His effect.

Again. Since He is an agent by will, He cannot do those things which He cannot will. Now we may realize what He cannot will if we consider how it is possible for necessity to be in the divine will: since what is of necessity is impossible not to be, and what is impossible to be, necessarily is not.

It is therefore evident that God cannot make Himself not to be, or not to be good or happy: because He necessarily wills Himself to be, and to be good and happy, as we proved in the First Book.

Again, it was shown above that God cannot will anything evil. Therefore it is evident that God cannot sin.

Likewise it was proved above that God’s will cannot be changeable: and consequently it cannot make that which is willed by Him, not to be fulfilled. It must however be observed that He is said to be unable to do this in a different sense from that in which He is said to be unable to do the things mentioned before. Because God is simply unable either to will or to make the foregoing. Whereas God can do or will these, if we consider His power or will absolutely, but not if we presuppose Him to will the opposite: for the divine will, in respect of creatures, has no necessity, except on a supposition, as we proved in the First Book. Hence all these statements, God cannot do the contrary of what He has decreed to do, and any like sayings are to be understood in the composite sense: for thus they imply a supposition of the divine will with regard to the opposite. But if they be understood in the divided sense, they are false, because they refer to God’s power and will absolutely.

And as God acts by will, so also does He act by intellect and knowledge, as we have proved. Hence He cannot do" what He has foreseen that He will not do, or omit to do what He has foreseen that He will do, for the same reason that He cannot do what He wills not to do, or omit to do what He wills. Also, each assertion is conceded and denied in the same sense, namely that He be said to be unable to do these things, not indeed absolutely, but on a certain condition or supposition.