In public political deliberation, people will err and lie in accordance with definite patterns. Such discourse failure results from behavior that is both instrumentally and epistemically rational. The deliberative practices of a liberal democracy (let alone repressive or non-democratic societies) cannot be improved so as to overcome the tendency for rational citizens to believe and say things at odds with reliable propositions of social science. The theory has several corollaries. One is that much contemporary political philosophy can be seen as an unsuccessful attempt to vindicate, on...