Doing What is Best

If doing what is best sets the right ideal of rational agency, then rational agents should do what they believe to be best. As long as we leave open the question what makes an option best, all plausible theories of rationality willfit into this framework. Consequently, rational agents will always want to do the things they believes to be best. This claim is an instance of what David Lewis calls the desire-as-belief thesis, which he has tried to refute. I reject Lewis' argument by criticizing his treatment of belief-change in respect of propositions about degrees of goodness.