 Consider the Prisoner’s Dilemma, a stock example of game theory. Two people isolated from each other may each act either cooperatively or uncooperatively. They each do better if they each act cooperatively than if they each act uncooperatively. However, each does better if he acts uncooperatively, no matter what the other does. Acting uncooperatively dominates acting cooperatively. Suppose, in addition, that the two players are psychological twins. Each thinks as the other thinks. Moreover, they know this fact about themselves. Then if one player acts cooperatively, he concludes that his counterpart also acts cooperatively. His acting cooperatively is good evidence that his counterpart does the same. Nonetheless, his acting cooperatively does not cause his counterpart to act cooperatively. He has no contact with his counterpart. Because he is better off not acting cooperatively whatever his counterpart does, not acting cooperatively is the better course. Acting cooperatively is auspicious but not efficacious.