The Uniqueness thesis: a hybrid approach

Uniqueness is the view that a body of evidence justifies a unique doxastic attitude toward any given proposition. Contemporary defences and criticisms of Uniqueness are generally indifferent to whether we formulate the view in terms of the coarse-grained attitude of belief or the fine-grained attitude of credence. This dissertation proposes and defends a hybrid view I call Hybrid Impermissivism, which combines the following two theses: Moderate Uniqueness and Credal Permissivism. Moderate Uniqueness says that no evidence could justify both believing a proposition and its negation. However, on Moderate Uniqueness, evidence could justify both believing and suspending judgement on a proposition (hence the adjective “Moderate”). And Credal Permissivism says that more than one credal attitude could be justified on the evidence. Hybrid Impermissisim is developed into a precise theory by using normative bridge principles on how rational belief and credence ought to interact. Hybrid Impermissivism is an attractive position in several respects: as I argue, it captures plausible motivating ideas behind permissive and impermissive epistemologies and avoids some important problems associated with them. Still, I show that Hybrid Impermissivism faces a special problem, the diachronic coordination problem, which has to do with coordinating an agent’s beliefs and credences over time. A significant part of this dissertation is dedicated to solving this new problem. I state a formal result I call the coordination theorem, which identifies plausible constraints on rational belief and credence under which Moderate Uniqueness and Credal Permissivism remain diachronically consistent. My overall conclusion is that Hybrid Impermissivism is a coherent, plausible, and conciliatory position and provides a correct characterisation of the requirements of evidence on doxastic attitudes.