  While Williams is commonly interpreted as challenging the possibility of an agent being motivated to do A by the belief that he has an external reason R to do A, on this reading he explicitly accepts that such motivation is possible; a disposition to be motivated by the belief that you have an external reason could be an element of your motivational set, making the fact that you have an external reason itself an internal reason for you to act. (It is an advantage of this interpretation that this is what Williams actually says.)