 The first question is, as Hardwig notes, part of a more general discussion about the epistemic value of testimony. Much of what passes for common knowledge is acquired from others. We depend on experts to tell us what is wrong or right with our appliances, our cars, our bodies. Indeed, much of what we later come to know depends on what we previously learned as children from our parents and teachers. We acquire knowledge of the world through the institutions of education, journalism, and scientific inquiry. Philosophers disagree about the status of beliefs acquired in this way. Here is the question: If A knows that p on the basis of evidence e, B has reason to think A trustworthy and B believes p on the basis of A’s testimony that p, does B also know that p? Some philosophers, as Locke and Hume seem to have, argue that only what one has observed oneself could count as a good reason for belief, and that the testimony of another is, therefore, never on its own sufficient warrant for belief. Thus, B does not know simply on the basis of A’s testimony but must have additional evidence about A’s reliability. While this result is consistent with traditional philosophical empiricism and rationalism, which emphasized the individual’s sense experience or rational apprehension as foundations of knowledge, it does have the consequence that we do not know most of what we think we know.