Vol.:(0123456789) 1 3 Topoi https://doi.org/10.1007/s11245-019-09678-x Epistemically Diferent Epistemic Peers Mariangela Zoe Cocchiaro 1,2  · Bryan Frances 3 © Springer Nature B.V. 2019 Abstract For over a decade now epistemologists have been thinking about the peer disagreement problem of whether a person is reasonable in not lowering her confdence in her belief P when she comes to accept that she has an epistemic peer on P who disbelieves P. However, epistemologists have overlooked a key realistic way how epistemic peers can, or even have to, difer epistemically—a way that reveals the inadequacy of both conformist and non-conformist views on peer disagreement by uncovering how the causes of peer disagreement bear on the debate’s core philosophical issue. Part of our argument for this thesis will involve giving a thorough yet entirely informal presentation of mathematical theorems in economics by Robert Aumann (Ann Stat 4(6):1236–1239,1976) and Polemarchakis and Geneakoplos (J Econ Theory 26:363–390,1982) which represent a formally precise description of how two rational agents must deal with disagreement under certain epistemically interesting circumstances. Keywords Peer disagreement · Peerhood · Aumann · Polemarchakis & Geneakoplos · Economics · Agree to disagree · Epistemic position · Ordinary disagreements 1 Introduction For over a decade now epistemologists have been thinking about the peer disagreement problem of whether a person is reasonable in not lowering her confdence in her belief P when she comes to accept that she has an epistemic peer on P who disbelieves P. However, epistemologists have overlooked a key realistic way how epistemic peers can, or even have to, difer epistemically—a way that reveals the inadequacy of both conformist and non-conformist views on peer disagreement by uncovering how the causes of peer disagreement bear on the debate’s core philosophical issue. Part of our argument for this thesis will involve giving a thor- ough yet entirely informal presentation of mathematical theo- rems in economics by Robert Aumann (1976) and Polemarcha- kis and Geneakoplos (1982) which represent a formally precise description of how two rational agents must deal with disagree- ment under certain epistemically interesting circumstances. 2 The Set Up Admitted peer disagreement in epistemology is often taken as evidence that at least one of the disagreeing parties is epistemically required to adjust her confdence in the propo- sition disagreed upon. In a diachronic fashion, at t 1 the two agents who accept each other as being peers on a proposition P each have a credence in P. At this point in time, each does not know the other person’s credence in P. By a later time t 2 the agents have discovered their difer- ent initial credences and they have shared any reasons or evidence they have regarding p: in other words, they have reached “full-disclosure” of the disagreement. 1 One of the primary normative questions at stake in the standard debate arises at t 2 and concerns how the admission of the disagreement, peerhood, and difering credences should afect what the agents think of P: are the agents reasonable, at that later time, if they stick with their initial credences in P? The philosophical literature ofers two general answers: * Mariangela Zoe Cocchiaro zoe.cocchiaro@yahoo.it Bryan Frances Bryan.frances@yahoo.com 1 University of Hong Kong, Hong Kong, Hong Kong 2 University of Salzburg, Salzburg, Austria 3 Institute of Philosophy and Semiotics, University of Tartu, Tartu, Estonia 1 This term comes from Feldman (2006).