Psychologism, Practical Reason and the Possibility of Error

Psychologism is the view that practical reasons are psychological states. It is widely thought that psychologism is supported by the following principle governing explanation:
TF. The difference between false and true beliefs on A's part cannot alter the form of the explanation which will be appropriate to A's actions. (TF) seems to imply that we always need to cite agents' beliefs when explaining their actions, no matter whether those beliefs are true or false. And this seems to vindicate psychologism. I argue, however, that the standard argument for psychologism which includes (TF) as a premise in fact fails to establish its intended conclusion. As a result, the overall case for psychologism is not as strong as it may initially seem.