Sorry, I don’t know much about the philosophy of mathematics, so your analogy goes over my head.
I don’t know what it means to add to that that the other player ’shouldn’t cooperate.
It means that it is optimal for the other player to defect, from the other player’s point of view, if they’re following the same decision rule that you’re following. Given that you’ve endorsed this decision rule to yourself, you have no grounds on which to say that others shouldn’t use it as well. If the other player chooses to cooperate, I would be happy because my preferences would have been fulfilled more than they would have been had he defected, but I would also judge that he had acted suboptimally, i.e. in a way he shouldn’t have.
Sorry, I don’t know much about the philosophy of mathematics, so your analogy goes over my head.
It means that it is optimal for the other player to defect, from the other player’s point of view, if they’re following the same decision rule that you’re following. Given that you’ve endorsed this decision rule to yourself, you have no grounds on which to say that others shouldn’t use it as well. If the other player chooses to cooperate, I would be happy because my preferences would have been fulfilled more than they would have been had he defected, but I would also judge that he had acted suboptimally, i.e. in a way he shouldn’t have.