In Hume’s moral theory, there is so much emphasis on the impotence of reason that one might probably think that, for Hume, reason has nothing to do with action and morality. Moreover, it is easy to lead us to the conclusion that passion is the only element in determining action, and moral sense or sentiment is the only element in distinguishing virtue from vice. According to this understanding, Hume’s dichotomy of reason and passion, or of reason and sentiment, is clear and distinct as well as his dichotomy of idea and impression. On the one side, reason is inert and impotent as well as idea; on the other side, passion and sentiment are full of activity and vivacity as well as impression.
However, with a little more careful reading, we find that Hume’s dichotomy is not so clear, and the role which he assigns to each side is not so convincing. Therefore, in this paper, I will discuss the distinction of reason and passion involved in his doctrine of action, and the dichotomy of reason and sentiment involved in his doctrine of moral distinction. The close relation of these two doctrines is the reason why I pick them up together. The moral distinction is supposed by Hume to cause passion， and then action.
I introduce Hume’s distinction of impression and idea first, because it is the basis of Hume’s theory and is involved in most of his important arguments. I discuss the dotrine of action prior to the doctrine of moral distinction, since several arguments in the latter presuppose the conclusions of the former.