To be responsible for their acts, agents must both perform those acts voluntarily and in some sense know what they are doing. Of these requirements, the voluntariness condition has been much discussed, but the epistemic condition has received far less attention. In Who Knew? George Sher seeks to rectify that imbalance. The book is divided in two halves, the first of which criticizes a popular but inadequate way of understanding the epistemic condition, while the second seeks to develop a more adequate alternative. It is often assumed that agents are responsible only for what they are aware of doing or bringing about--that their responsibility extends only as far as the searchlight of their consciousness. The book criticizes this searchlight view on two main grounds: first, that it is inconsistent with our attributions of responsibility to a broad range of agents who should but do not realize that they are acting wrongly or foolishly, and, second, that the view is not independently defensible. The books positive view construes the crucial relation between an agent and his failure to recognize the wrongness or foolishness of what he is doing in causal terms: the agent is responsible when, and because, his failure to respond to his reasons for believing that he is acting wrongly or foolishly has its origins in the same constitutive psychology that generally does render him reason-responsive.