  Unfortunately, the fact that an agent’s belief that R is an external reason to do A can motivate her to do A does not suffice to show that R is a reason to do A. It only shows that the fact that R is a reason to do A is a reason to do A. That is because it does not show that R can explain the agent’s motivation to herself; it only shows that the fact that R is a reason for her to do A can explain her motivation to herself. So what Williams wants to know is, how could it be true that R is a reason to do A? If it were true, realizing that it was could motivate—but what could make it true?