What are possible worlds?

If God is freely creative, then outside him there exists nothing except what he freely chooses; there are not even possibilities, there is nothing at all…McTaggart’s argument…may fail to strike people because they have some false imagination of God’s being confronted by a magazine of possible entities, between which he has to choose. This is an old error, on which I have spoken and written before; I shall not now waste time discussing it; enough to say that the phrase “the possibility of a rational cephalopod” (for example) can only relate to the same as the phrase “the possibility of God’s making a cephalopod rational,” and this in turn is a nominalization of the proposition (be it true or false) “God can make a cephalopod rational.” We have to do only with some aspect of God’s almighty power, not with some possible but unactual rational cephalopod or with some logical fact that confronts God. P. Geach, The Virtues (Cambridge: Cambridge University Press, 1977), 73-74.