Law depends for its existence and practice upon vast concepts that stretch across time, space, causation, and agency. Vast concepts are fundamental from legislation and interpretation to enforcement and adjudication; from weighing evidence to establishing motive and intent; and from imposing fines or sentences to awarding compensation. But all of human thought and memory is just here and now. Forming and understanding vast legal concepts can be difficult, and failures arise in theory and practice when legal concepts fail to certain constraints on intelligibility, congeniality, efficiency, and memorability. We present a model of the formation of legal concepts and some constraints on their practical utility. We also suggest a research agenda that may allow us to better understand what sorts of legal concepts work, and which ones we discard and when.