This paper argues that we need to distinguish between two different ideas of a reason:first,the idea of a premise or assumption,from which a person's action or deliberation can proceed; second,the idea of a fact by which a person can be guided,when he modifies his thought or behaviour in some way.It argues further that if we have the first idea in mind,one can act for the reason that p regardless of whether it is the case that p,and regardless of whether one believes that p.But if we have the second idea in mind,one cannot act for the reason that p unless one knows that p.The last part of the paper briefly indicates how the second idea of a reason can contribute to a larger argument,showing that it is better to conceive of knowledge as a kind of ability than as a kind of belief.