Nagel is a skeptic when it comes to moral responsibility. He notes the criterion for determining whether an action is good or bad is based on luck, then it is not our choice to be good or bad. In other words, it is impossible to be morally responsible if the only way to tell if an action is good or bad is to look at the consequences determined by luck. If luck determines the rightness or wrongness of an action, then no person is responsible for the consequences of his or her actions.
Sven notes that there are possible responses to this kind of skepticism. We can, like Frankfurt, say that moral responsibility does not depend on free will and control of consequences. Also, we can separate the moral worth of an action from the means of assessing the moral worth of an action. Someone may object to Nagel with an epistemic response to the problem of moral luck. Basically, there may be times that we are unable to know why a person's action is morally good or bad but this does not change the fact that the action is good or bad. Luck does not affect the morality of an action. Luck only affects how or whether I know if an action is morally good or bad.