What if anything justifies us in believing the testimony of others? How should we react to disagreement between ourselves and our peers, and to disagreement among the experts when we ourselves are novices? Can beliefs be held by groups of people in addition to the people composing those groups? And if so, how should groups go about forming their beliefs? How should we design social systems, such as legal juries and scientific research-sharing schemes, to promote knowledge among the people who engage in them? When different groups of people judge different beliefs to be justified, how can we...