It is often assumed that democracies can make good use of the epistemic benefits of diversity among their citizenry, but difficult to show why this is the case. In a deliberative democracy, epistemically relevant diversity has three aspects: the diversity of opinions, values, and perspectives. Deliberative democrats generally argue for an epistemic form of Rawls' difference principle: that good deliberative practice ought to maximize deliberative inputs, whatever they are, so as to benefit all deliberators, including the least effective. The proper maximandum of such a principle is not the pool of reasons, but rather the availability of perspectives. This sort of diversity makes robustness across different perspectives the proper epistemic aim of deliberative processes. Robustness also offers a measure of success for those democratic practices of inquiry based on the deliberation of all citizens.