Could God create a stone so heavy He couldn't lift it?

Believers in God generally agree that God is omnipotent or all-powerful, or able to do all things. But it's also widely held that God is also perfectly good, which would seem to mean that there is something He cannot do, namely sin. It further seems plausible to believe that not even God could change the past, since it no longer exists and is therefore not around for any causal powers to affect it. And then there is our title question, versions of which have been around for centuries, which seems to admit only two possible answers: either God could, or could not, create such a stone. But if He could, then there could be something God cannot do (namely lift that stone); and if He could not, then there already is something God cannot do (namely create that stone). Any way you think about it, it seems there are or could be things God cannot do: sin, change the past, lift or create that stone. And how could God be omnipotent if there could be things He cannot do?

To answer, one must get clearer on what it means to say that God is omnipotent.

The great medieval theologian, St. Thomas Aquinas, suggests it’s this: that God can do all things that are possible. But which things are these, exactly? Well, Aquinas says, something is said to be possible in two senses: either in relation to or for some power, or absolutely, in itself. Lifting a spoon is said to be possible for human beings because we have the power to do so; and it is possible absolutely, in itself, because the idea of “lifting a spoon” doesn’t involve any contradictions. Now we shouldn’t say that God can do all things that are possible in the first sense, in relation to His own power, since that amounts merely to saying that He can do all the things He has the power to do –- which is true but rather uninformative. God’s omnipotence should therefore be understood in the second sense, as His ability to do everything that is possible absolutely, in itself; that is, to do everything that is logically possible, that does not involve a contradiction.

Some may feel that this is not enough; that God’s power is such that He can do not only everything logically possible but even everything logically impossible –- that is, even things involving contradictions. But though this sounds impressive it actually doesn’t say very much. Consider some contradictory thing, such as a round square. You can form the phrase “round square,” but if you think about it you’ll realize that the phrase lacks any real meaning: if something is round it’s not square, and if it’s square it’s not round. To insist that God could make a round square is therefore to utter a literally meaningless sentence; it’s not really to add anything to His power. And if the alleged ability to do this contradictory thing doesn’t really add to God’s power, then neither does the inability to do it detract from His power.

Now understanding that God’s omnipotence is His ability to do all logically possible (non-contradictory) things, Aquinas may resolve our puzzles. No, God cannot sin. But to sin is to fall short in some way; it is to be limited in one’s power to do as one should. Obviously an all-powerful being would be unable to do this, since His very power ensures that nothing could limit His power. So the idea of an all-powerful being sinning is the idea of an all-powerful being that is limited in its powers –- a contradiction. God’s inability to sin is an inability to do the impossible, so it’s not, therefore, a genuine limit on His power.

Similarly, that something is past means it occurred; to change it would mean that it didn’t occur, which contradicts its occurring. So expecting God to be able to change the past –- or create a stone so heavy an omnipotent being like Himself couldn’t lift it -– is like expecting Him to create a round square. He can’t do any of these. But that’s not because His power –- to do all possible things –- is limited, but because none of these is, in itself, a logically possible thing to do.


Source: St. Thomas Aquinas, Summa Theologica I.25.3