I like to use the stone argument as a warm-up in a philosophy of religion class. But it's actually kind of tricky to use. Here's a natural way to put it:
- Either God can or cannot make a stone he can't lift.
- If God cannot make such a stone, then there is something God can't do.
- If God can make such a stone, then there is something God can't do, namely lift the stone.
- So, there is something God can't do.
But in this formulation, (3) can be easily rejected. It does not follow follow from God's merely being able to make such a stone that there is something God can't do, just as it doesn't follow from God's being able to make a unicorn that there is a unicorn. The correct conditional is:
- If God does make such a stone, then there is something God can't do, namely lift the stone.
This means that the stone argument isn't actually an argument against omnipotence. If all that was in view was omnipotence, one could say: "Sure, God can create such a stone. Were he to create it, he wouldn't be omnipotent. But he hasn't created such a stone and he is omnipotent." Rather, we should take the stone argument as an argument against essential omnipotence. And that makes the argument a little less suited for warm-up classroom use, because one has to introduce the notion of an essential property.
What I actually did in class today is I gave the argument in the invalid form. Alas, nobody caught the invalidity. Though, interestingly, one student was unsure of disjunction-elimination in general.
I also emphasized that the stone wasn't really a problem for omnipotence, but for particular attempts to define omnipotence. I think it's important to to distinguish those atheological arguments that are problems for theism from those that are problems for particular ways of defining theism. The inductive problem of evil is an argument against theism; the stone argument is only an argument against particular formulations.
