What is the rational response to disagreement with an epistemic peer? Some say the steadfast response of holding on to your own belief can be rational; others argue that some degree of conciliation is always rationally required. I argue that only an epistemological externalist about rationality-someone who holds that the rationality of a belief is partly constituted by factors outside a subject's cognitive perspective-can defend the steadfast view. Or at least that this is so in the kinds of idealized cases of peer disagreement that take center stage in the current debate about disagreement. The argument has three steps. First, I show how rationality internalism motivates conciliationism: in view of the mutually recognized internal epistemic symmetry between peers, it would be arbitrary for either peer to hold on to her own belief. Second, I strengthen this line of thought by considering various proposed 'symmetry breakers' that appear to introduce a relevant asymmetry between peers, which could be used to defend the rationality of a steadfast response. I argue that none of these alleged symmetry breakers can help internalists. Third, I show how externalism does have the resources to defend steadfastness and expose how extant defenses of steadfastness implicitly rely on externalist intuitions. © 2013 Springer Science+Business Media Dordrecht.