Directions of fit and the humean theory of motivation 1

According to the Humean theory of motivation, a person can only be motivated to act by a desire together with a relevantly related belief. More specifically, a person can only be motivated to ? by a desire to ψ together with a belief that ?-ing is a means to or a way of ψ-ing. In recent writings, Michael Smith gives what has become a very influential argument in favour of the Humean claim that desire is a necessary part of motivation, and a great deal has been written about Smith's defence of this Humean claim. However, no one has yet identified the fundamental weakness of his defence. The fundamental weakness is that there is no single conception of directions of fit that does all the work Smith needs it to do throughout the various stages of his defence.