- Either (a) one cannot desire what one does not believe to be worth having or (b) it is possible to have no reason, even prima facie, to bring about something one desires.
A simple case is where you are told you will avoid great punishment if you induce in yourself a desire to eat mud, which you antecedently know there is no point to doing. You then induce in yourself this desire. If (a) is false, you should be able to do this while maintaining your belief that there is no point to the action. But you do not gain a reason to do the action in this case: you only have a reason to desire it, not a reason to do it.
Objection: There is inner turmoil when you have an unsatisfied desire for eating mud.
Response: This isn't a reason to eat mud, as much as a reason to end having a desire to eat mud. Eating mud is sometimes a means to this, but not always. We could imagine a situation where the only ways available to eat mud are ways that wouldn't make the desire go away (e.g., you know you have a psychological quirk that makes the desire not go away, or maybe you know an alien will ensure that as soon as you swallow mud, you start believing you're eating chocolate, so you will never form the belief that you have eaten mud, though you will have).