I've posted some of this over at Ethics Etc., but as the thread is dead I thought I'd post a little on the matter here. Daniel Star and Stephen Kearns have written a few papers explaining and defending the view on which normative or justifying reasons are pieces of evidence about what we should believe or should do. It's an interesting view and the arguments they offer appeal to a number of considerations I'd appeal to in arguing that pieces of evidence do not constitute reasons in the relevant sense.
The first worry, which I posted over at Ethics Etc., went something like this. I believe it was Bernard Williams who observed that there can be a kind of conflict among practical reasons that there cannot be among theoretical reasons. If we identify both reasons for action and belief with evidence, I think you’ll have to deny Williams’ observation. So, the observation. You can judge that you ought to X knowing that there are real reasons not to that serve as reasons for regret. Moreover, you can _knowingly_ judge that you ought to X while knowing that there are, as it were, real reasons not to X that are defeated by reasons that favor X-ing. However, if you judge that you ought to believe, you cannot knowingly judge that you ought to believe while knowingly judging that there are real reasons not to believe. The evidence that speaks against believing would be regarded as misleading. I can’t see how this comes out true on a view on which all reasons are pieces of evidence.
The second worry concerns the status of the following thesis:
(OR) If S ought to X/S ought not X, there is a reason for S to X/there is reason for S not to X.
I don't see why we'd say the following is true:
(OE) If S ought to X, there is some evidence that S ought to X/there is evidence that S ought not X.
But, if Kearns and Star are right, I don't see how OE could not be a consequence of OR. Maybe there are good reasons to accept (OE), but I really cannot think of any offhand. Consider cases in which the subject's wrongful action is due to a non-culpably mistaken belief. To say that the beliefs are non-culpably mistaken is to say that they are the proper response to the evidence _available_. But, to say that the action was wrongful was to say that there were reasons not to do it. It seems that what makes the action wrongful is a set of facts that does not strongly supervene on the evidence at hand. To accommodate such cases, it seems that Kearns and Star would have to say that the sort of evidence they have in mind is not necessarily evidence possessed but evidence there is. I'm not sure how to make sense of evidence possessed or not that would not trivialize (OE) so that it just becomes the view that if you ought to do something there are some facts in the world in light of which this is true.