Wouldn't the lie you tell yourself in the situation be the best summary of useful behavior, or a utility function, AS a statement of fact? The best way to act is not in any way a statement of fact; but to represent to ourselves a situation and our best course of action in it, we must frequently encode the information as a set of facts. As you say, a full game-theoretical expose of how I should ACT is not possible. Therefore, I represent to myself some useful idea of what the situation IS, not based on its accuracy, but on its utility. This is an AS IF argument.