The title of my paper, "Action and Reason"is intended to call attention to one of the main problems, if not the main problem of practical philosophy. The subtitle, "Aristotle vs. Kant" places the theme in the context of a fictitious controversy in the history of philosophy. I want to discuss a question which is so fundamental that the very possibility of a practical philosophy as such would seem the depend upon its resolution. But I hope to become clearer about just what difficulties are involved in the problem of action and reason, with the help of some historical background.