  According to this reading, the problem Williams sees for external reasons is the following. For there genuinely to be external reasons, he observes, it must be possible that some such external reasons beliefs are true. This requires that the consideration R which an agent accepts as his reason must actually be a genuine explanation of his acting under the condition of sound deliberation, independently of any facts about his motivational set.  But this condition cannot be met, because nothing could be explanatory of an agent’s action independently of the contents of his motivational set: his desires and dispositions. From this it follows (given the conceptual premise) that no motivationally ‘external’ considerations could genuinely be practical reasons for an agent.