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.