  The weakest point in this version of Williams’ argument is probably its fundamental, conceptual premise: that the concept of a practical reason is the concept of an explanation of action under certain conditions. Even if we grant the controversial claim that the concept of a practical reason is the concept of an explanation we can still resist this analysis. Suppose, for example, that the concept of a reason to do A is the concept of an explanation of why to do A, or of why doing A is a good thing to do. To say that R was the reason for which the agent did A would then be to say that R was the explanation of why to do A which motivated the agent to do A. This rival account respects the conceptual relation between ‘reason’ and ‘explanation’ on which Williams and Davidson insist, but doesn’t analyze practical reasons as any kind of explanation of action. If this is what our concept of practical reasons is, then a different argument will be needed if we are to rule out the possibility of external reasons.