Thesis advisor: Daniel J. McKaughan / When you learn that you disagree with an epistemic peer, what should you believe about the proposition you disagree about? The epistemology of peer disagreement has made considerable progress in answering this question. But to this point, we have largely neglected a significant resource which can help us to determine how peers who disagree can rationally respond to their disagreement. Closely examining actual disagreements in scientific and nonscientific contexts can help us to understand why peers find themselves in disagreement. And knowing why you disagree with your peer can help you to understand how you can rationally respond to your disagreement. Examining specific scientific and nonscientific disagreements shows us that some peers disagree because they disagree about what evidence is relevant to the proposition they disagree about. Dual disagreements about propositions and evidence can be found in numerous areas of disagreement, including ethical, political, philosophical, and scientific arenas. When you find yourself in these dual disagreements, you can rationally believe that your belief is rational and that your peer’s belief, though it diverges from yours, is also rational. But some philosophers have suggested that this situation in which you and your peer have rational beliefs and recognize each other as holding rational beliefs is impossible. A primary motivation for thinking that at least one of you must be believing irrationally is the thesis of Uniqueness about rationality, which states that at most one doxastic attitude can be rationally held given a body of evidence. However, when you consider the epistemic context of your actual disagreements with your peer carefully, you need not think that at least one of you is believing irrationally, even if Uniqueness is true. In response to your disagreement with a peer who disagrees with you both about what evidence is relevant to the proposition you disagree about and the proposition itself, you can even rationally hold a belief which splits the connection between your evidence and your evidence about your evidence. When we consider their epistemic contexts in full relief, peers in disagreement can simultaneously be believing rationally, even if only one of them is right. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Philosophy.