Korsgaard's view is basically that a reason is a consideration in favor of doing something. The consideration is provided by a proper combination of both the end and what one is to do in order to achieve that end. (In her terminology, what one does is an act, and the action as a whole involves an-act-for-the-sake-of-an-end.) In asking for a reason, then, we are asking for a description of the proposed (or performed) action such that both the act and the end are specified in such a way as to make the action as a whole appear worth performing to the agent.
As she writes in "Acting for a Reason" (printed in The Constitution of Agency):
If Aristotle and Kant are right about actions being done for their own sakes, then it seems as if every action is done for the same reason, namely because the agent thinks it's worth doing for its own sake. This obviously isn't what we are asking for when we ask for the reason why someone did something, because the answer is always the same: he thought it was worth doing. What may be worth asking for is an explication of the action, a complete description of it, which will show us why he thought it was worth doing. (221)And later:
Aristotle and Kant's view, therefore, correctly identifies the kind of item that can serve as a reason for action: the maxim or logos of an action, which expresses the agent's endorsement of the appropriateness of doing a certain act for the sake of a certain end. (226)Now I wonder if Korsgaard has any means at all of accommodating any sort of externalist view of reasons. Reasons are, on her account, entirely up to the agent: a reason gives a description of the action such that it makes the action appear worth doing to the agent (or, to put it another way, it gives a description of the action such that the agent is motivated to perform that action). But on her account, as far as I can tell, there is just no grounds at all for saying something like this: "John has a reason to push that button, even though he doesn't know it." That is, on her account--from what I can tell--a consideration can only be a reason if it is taken as such by an agent.
I suppose there is a way of fixing this. One could say that a reason is either a consideration that motivates A (or makes the action appear worth doing to A), or it is a consideration that would motivate A, were he fully aware of the relevant facts. Similarly, one would have to add: Even when A takes something to be a reason for him, it may still not be a reason. For example, John might believe that pressing the button will launch a bomb, and so he has a reason not to press it. But in fact pressing the button will stop the bomb from being launched, so what he takes to be a reason isn't a reason at all. But I suspect Korsgaard does not want to go in this direction: this is why she refers, in the second quote above, to "the kind of item that can serve as a reason for action." If I am reading this correctly, then, the fact that pressing the button will stop the bomb from being launched will enter into "kind of item that can serve as a reason" for John, but it is not a reason for John. And that seems wrong, for if John were aware of the button's function, he would recognize it as a reason, and this suggets that it is a reason for him, albeit one he does not have access to.
Her account, then, seems to be far more internalist than the one proposed by Williams ("Internal and External Reasons"). Williams, after all, recognizes that something is a reason for an agent so long as there is a path to it from the agent's subjective motivational set. But Korsgaard seems to reject this requirement: unless something is taken as a reason, it doesn't seem to be a reason at all.
In other words, I think Korsgaard's account as given is false: just because an agent takes something to be a reason does not make it a reason at all (and the fact that he fails to take something as a reason does not mean that it is not). What makes it a reason is that he could take it to be a reason, were he fully informed. (Like Setiya, then, Korsgaard portrays reasons as supervening on the agent's mental states, but she doesn't even add the proviso that none of his beliefs may be false, the way Setiya does.)