Introduction

Almost every society exhibits diversity. People have diverse beliefs and desires, based on diverse information and interests. This empirical fact poses several challenges. First, how can, and should, individuals respond to the diversity of opinion among their peers? Second, how can, and should, society be organized for collective decision-making in the face of diversity? And third, how can, and should, political arrangements and public policies be justified in a diverse society? This issue brings together papers by eight leading scholars addressing these questions. The papers fall into three groups, roughly corresponding to the three questions just raised: the first group focuses on rational aspects of diversity, the second on institutional aspects, the third on normative ones.

Let me briefly review their contributions. What challenges does diversity of opinion pose for rationality? Richard Bradley argues that it creates both problems and opportunities: it makes it harder to achieve consensus on important matters, yet it enables individuals to improve their opinions. Bradley identifies a tension between rational opinion revision at the individual level and rational opinion pooling at the collective one. Solving this tension, he argues, requires a departure from classical methods of opinion pooling. Philip Pettit contrasts two kinds of evidence we may consider in our quest for consensus on a given matter: substantive evidence on the one hand and formal testimonial evidence on the other, for example the fact that a majority holds a particular opinion. Is it rational to defer to the latter kind of evidence? Pettit argues that deference to formal testimonial evidence is not a generally viable route to consensus, because it may lead to inconsistent opinions. Drawing on the emerging theory on judgment aggregation, he points out that majority opinions can be logically inconsistent even when all underlying individual opinions are perfectly consistent. How to deal with testimonial evidence is also the subject of Robert Goodin's paper. While it is widely acknowledged that we can learn much from the reports of unbiased observers, Goodin asks whether we can learn anything from those of biased ones. He argues that, surprisingly, we can learn a fair amount, provided the reports go against the grain of the observers' biases. A report warning about the serious dangers of smoking, for example, may well be persuasive when it comes from a scientist favourably disposed to the tobacco industry. Goodin suggests that, under certain conditions, we can triangulate on the truth by pooling the reports of multiple biased observers.

The challenges diversity of opinion poses for institutional design are the topic of the next group of papers. James Bohman notes that, although it is often assumed that democracies benefit from diversity, it is not so easy to show how exactly they do so. Focusing on deliberative democracy, Bohman offers an account of how to make the best use of epistemic diversity. Adapting an epistemic form of Rawls's difference principle, he argues that good deliberative democratic institutions and practices are ones that maximize the availability of different perspectives so as to benefit all deliberators, including the least effective. Cass Sunstein questions some of the widely accepted advantages of deliberative arrangements, arguing that deliberating groups often converge on falsehood rather than truth, by amplifying individual errors, emphasizing shared information while ignoring private one, and falling victim to informational and reputational cascades or group polarization. Sunstein suggests that suitably designed prediction markets avoid some of these pitfalls and concludes that their success gives us important insights into how a group can pool the diverse information of its members. Josiah Ober presents a historical case study of how the community of classical Athens managed to pool its citizens' knowledge so as to achieve common knowledge for solving various coordination problems. By analysing a law-court speech delivered by a prominent Athenian leader, Ober uncovers how Athenian democratic institutions and practices promoted common knowledge. He shows that repeated public rituals, public monuments and public architecture were among the key mechanisms, since they allowed for the shared presence and interaction of a large number of citizens.

The final two papers discuss the challenges diversity of opinion poses for political justification. Daniel Weinstock begins by observing that much of...