But the conditions were not defined that this is his action, his action/ his verb (thats whats happening ) is that he is unable, so he is capable of not being capable of lifting the stone which is in itself the "screwy-ness," of the paradox. If it said that god said he would not pick it up you would be 100% correct, but because his action is to not be capable of the other action, he is an entity by definition, so he as an individual is incapable. If he could it would mean he is capable of being impossible to his own rules. Which as god by definition would be possible, because he can do the