What is the relation between knowledge and action? Accord ing to one standard picture, there is none. Rational action is a matter of maximizing expected utility, where expected utility is a function of utility and subjective credence. It is subjective degrees of belief that matter for rational action, not knowledge. On this picture, having knowledge that p is independent of whether it is rational to act on one's belief that p: knowledge that p is not sufficient since one may know that p despite lacking sufficiently high subjective credence to warrant acting on the proposition that p; and knowledge that p is not necessary, since high subjective credence can rationalize action even in the absence of knowledge. In contrast to the picture just sketched, our ordinary folk appraisals of the behavior of others suggest that the concept of knowledge is inti mately intertwined with the rationality of action. Suppose, for example, that Hannah and Sarah are trying to find a restaurant, at which they have time-limited reservations. Instead of asking someone for direc tions, Hannah goes on her hunch that the restaurant is down a street on the left. After walking for some amount of time, it becomes quite clear that they went down the wrong street. A natural way for Sarah to point out that Hannah made the wrong decision is to say," You shouldn't have gone down this street, since you did not know that the restaurant was here." Here is a similar case. Suppose John decides not to buy health insurance anymore, reasoning that he is healthy enough.

* The authors of this paper are listed in alphabetical order. We have given this paper at the Rutgers/Princeton Graduate Conference, Oxford University, University of Edinburgh, St. Andrews University, University of Calgary, Harvard University, and the University of Aberdeen. We are grateful to all the members of the audience who partici pated at those occasions. Keith DeRose, James Pryor, and Stephen Schiffer gave us help ful comments on written versions of the paper. Finally, as usual, the greatest thanks are due to Timothy Williamson, who provided both oral and written commentary at various stages throughout the process.