Abstract: By "lay Justices" I mean Justices of the Supreme Court of the United States who are not accredited lawyers. Currently the number of lay Justices is zero, although there is no constitutional or statutory rule that requires this. Commentators who urge that the Supreme Court should be diverse on all sorts of margins—methodological diversity, ideological diversity, and racial or ethnic or gender diversity—say little or nothing about professional diversity on the Court. I shall suggest that the optimal number of lay Justices is greater than zero, under specified empirical conditions. I do not know whether those conditions actually hold, but on the other hand no one knows that they do not. It is very plausible that the conditions do hold, in which case the status quo of zero lay Justices is an implausible extreme. In the strong form of the argument, it would be a good idea (whether or not it is a politically feasible one) to appoint a historian, economist, doctor, accountant, soldier, or some other nonlawyer professional to the Court. In a weaker form of the argument, I also suggest that at a minimum, we should appoint more dual-competent Justices—lawyers who also have a degree or some other real expertise in another body of knowledge or skill...