Where Concepts Come Apart

Philosophy's central problems are notorious for their persistence. Two and a half millennia after Plato's Republic, the questions of what knowledge is, whether we act freely, what makes a person the same person over time, and what it is for one thing to cause another remain contested, and survey evidence suggests that the distribution of professional opinion on them barely moves across a decade. Some conclude that philosophy makes no progress; others redefine progress so that it does. This dissertation offers a third response.

It argues that the concepts at the centre of philosophy's persistent problems are polyfunctional: a single concept does several jobs in our practices, each job imposes its own application condition, and the conditions coincide extensionally across the range of cases from which the concept is learned. Philosophical problems arise where these conditions come apart. The thought experiment is the instrument by which such divergence cases are constructed in advance of the world delivering them.

On this account the familiar structure of a philosophical problem, a set of individually compelling but jointly inconsistent claims, is explained rather than merely described: each claim articulates one function of the concept, and each is compelling because the concept really does perform that function. Doctrinal positions are prioritisations of functions. The residual disagreement, once factual and merely verbal components have been removed, is a normative disagreement about which function the shared concept should serve in the divergence region. That disagreement is rationally stable for the same reason that reasonable disagreement about the weighting of plural values is stable, and its persistence is therefore not a scandal.

Progress, on this account, consists in the discovery of divergence, the articulation of functions, their disentanglement into distinct concepts, and the relocation of the residual question to the normative domain, where it can be argued as the practical question it is. The account is developed through four case studies (knowledge, free will, personal identity, causation), defended against objections including a reflexive application to the concept of progress itself, and shown to predict the pattern of stability and variance reported in experimental philosophy.