Experts often disagree on major issues within their fields of inquiry, despite being epistemic peers, individuals who are equally well-informed and equally well-equipped to correctly assess those issues. If these peers are familiar with each other’s expertise and the reasons each has for holding an alternative view, can they reasonably maintain their disagreement or should they conclude that at least one party must have been unreasonable in drawing the conclusion that she did? On the one hand, it would also be strange to conclude that, as experts, they have unreasonable beliefs in their areas of expertise. And yet, if they indeed recognize each other as epistemic peers, then it seems to fly in the face of rationality that either could believe she is more likely to be right than the other. They are equals in all the relevant ways.

Responses in the literature to this type of scenario have trended two ways: to the negative, where it is argued that reasonable disagreement amongst epistemic peers is not possible and therefore the peers should accept one of the conclusions or suspend judgment; and to the positive, where it is argued that peers can reasonably hold steadfast in their beliefs despite peer disagreement. I side with the steadfast view. In particular, I suggest that the method of inquiry known as reflective equilibrium (RE) allows for such a response to peer disagreement. RE has hitherto received less attention in epistemology than elsewhere in philosophy, e.g., ethics and social-political philosophy. Focusing on the underlying epistemic features of this method of inquiry, I defend an evidentialist interpretation of RE. From this interpretation, I then argue that RE is well-equipped to deal with many of the challenges of peer disagreement, revealing the possibility for reasonable steadfast responses to the discovery of a disagreeing peer. The steadfast response to peer disagreement does not thereby allow for peers to reasonably disagree, however. Rather, RE highlights how inquiry can advance while facing the rationality constraints offered in a case of peer disagreement.

The possibility of reasonable peer disagreement has extensive consequences for individual rationality and social epistemology. If we can get clear on the evidential factors relevant to disagreement, then we are better placed to recognize salient and irrelevant social factors in a specific disagreement. My evidentialist conception of RE clarifies the factors that can lead rational peers to draw opposite conclusions from a body of evidence. These conclusions may be doxastically blameless, while also being evidentially mistaken. That is, one may believe on what one takes to be good reasons, but is not in fact epistemically justified in doing so. There is a tension, then, between what I argue is the most rational belief forming process, RE, and our abilities to be perfectly epistemically rational. This tension is the result of our limitations as agents who cannot reason independently of our own perspectives. Employing RE to address issues in the epistemology of disagreement reveals that what might be egocentrically rational for one to believe does not guarantee that one is also epistemically rational in holding that belief.

I defend RE’s status as a rational method of inquiry and explore the impacts that RE and peer disagreement rationally have on each other over six chapters, divided into two parts: Chapters 1 – 3 develop and defend an evidentialist interpretation of RE according to which RE guarantees reasonable beliefs. Chapters 4 – 6 focus on challenges from and responses to issues in the reasonable disagreement debate using my evidentialist interpretation of RE.

Chapter 1 traces the history of RE, from John Rawls’s and Nelson Goodman’s early use to contemporary applications in moral theory and epistemology. I propose here that the best way to interpret this concept is evidentially. If RE produces justified beliefs, then it does so because reaching reflective equilibrium with a set of beliefs requires that all beliefs within that set are based on one’s total evidence.

Chapter 2 expands the conditions needed for a set of beliefs to be in reflective equilibrium, ultimately settling on four: a reflective component; an equilibrium or coherence component; a width condition that that requires one to consider the leading views of others working in the same domain; and a comprehensiveness condition that ensures we are capturing what RE is after—viz. providing a method that looks at an entire topic, ensuring that the beliefs within that topic are reasonable in light of each other and not just reasonable in light of some set of beliefs. This chapter develops the details and motivation for each of these conditions.

Chapter 3 argues that, contrary to several challenges, RE guarantees both epistemically and interpersonally reasonable beliefs. RE guarantees epistemically reasonable beliefs in virtue of the evidential requirements required of the agent satisfying RE—one cannot satisfy RE without also satisfying the evidentialist conditions for epistemic justification. RE guarantees interpersonally reasonable beliefs in virtue of the width condition. Due diligence in one’s consideration of alternatives to one’s own judgments provides a means of avoiding isolation concerns, while also being recognizable to third parties as a rational and epistemically responsible process.

Chapter 4 addresses a challenge to RE from the reasonable disagreement debate: How should one rationally respond to the discovery of a disagreeing peer? Applying this question to RE, we ask: Is there a normative application to the discovery of disagreement embedded in RE? I argue that there is no special status given to disagreement, other than the changes one will make in regaining equilibrium based on the evidence received from opposing judgments. So, while one’s doxastic state will almost always be adjusted, RE does not require that one adopts an entirely new doxastic attitude, or even brings oneself closer to the alternative judgment. The only mandate on this view is that one believes based on one’s evidence, which has nothing to do with disagreement specifically.

Chapter 5 considers a second challenge emerging from the epistemology of disagreement: Is reasonable peer disagreement possible under full disclosure? Under the assumption that RE produces reasonable judgments, I apply it to extant positions within this dialectic. It is once again argued that RE is consistent with views defending both the possibility and the impossibility of reasonable peer disagreement. In making the case for this conclusion, I consider how the conditions for RE could be undermined by one’s participation in a peer disagreement. Conversely, I also consider two ways in which RE might allow for such disagreement.

Chapter 6 further considers the rational impact RE and reasonable peer disagreement have on each other. Unlike the prior chapter, it is not assumed here that RE guarantees reasonable judgments. Instead, we consider three options concerning rationality at the intersection of RE and the epistemology of disagreement. The first option is that one cannot satisfy the conditions for RE while one is involved in a peer disagreement under full disclosure. The second option is that RE can be satisfied by peers who are in a disagreement, which shows that reasonable peer disagreement is possible. And the final option is that RE allows for peers to disagree, which shows that it cannot guarantee reasonable judgments. It is argued that the first option is the most promising: peer disagreement undermines one’s ability to satisfy RE, but that when one does satisfy these conditions, one is doxastically blameless