In this case study I argue that experts, to gain relevance in a jury trial, need to fit into a manifold division of knowing. They do so by borrowing and sharing diverse knowledges. These exchanges place the modest expert testimony right into an authoritative and powerful decision-making apparatus. This argument derives from an ethnographic study of a ‘‘sleepwalking defense.’’ The division of knowing embraces the certified facts, the instructed case, the competing expertise, and the common sense. As a conclusion, I (...) identify the experts’ twofold relevance. Experts perform the case as undecided and decidable. They provide exclusive knowledge and affirm a set of other knowledges. By ‘‘knowing’’ and ‘‘not knowing,’’ the experts perform individual modesty and systemic immodesty by the same token. (shrink)