There is a story behind our concepts, a history of how and why we came to think as we do. But do the origins of our concepts matter for our understanding of their current content and value? Philosophy has tended to be neglectful of the history of the concepts it tries to understand. Yet in recent years, the humanities and the sciences have seen an explosion of genealogical reasoning, which infers from the genesis or causal history of something to its content or validity. This reinvigorates the question of what genealogical reasoning can do, a question that is rendered all the more acute by the striking lack of systematic attempts to answer it. The overarching purpose of this PhD project in philosophy is to address this lacuna and demonstrate the legitimacy and fruitfulness of a genealogical approach to concepts. This is of particular importance to the humanities, which have of late been in the grip of a dichotomy between historicist constructivism/relativism and ahistorical absolutism. Part of this project’s value lies in revealing that dichotomy to be false by offering genealogy as a third way. One of the animating ideas of the project is that genealogy can be usefully combined with recently prominent externalist theories of content, which hold that the content of what we think and say depends on factors external to us and may therefore not be transparent to us. Taking the relations articulating content to be dynamic and diachronic, this project develops an alternative to ahistorical conceptual analysis, which reconstructs how a concept was synthesised in order to determine why it developed in the first place, thereby allowing us to contrast the functional core of the concept with its more contingent features. Moreover, this method renders intelligible how history can be deployed to criticise and vindicate the way we think and act. While it is a common complaint against genealogical reasoning that it commits either the genetic or the naturalistic fallacy, this project aims to show that these charges can be resisted, and promises a differentiated understanding of the conditions under which genealogical reasoning is legitimate.The project has five stages. Setting up a contrast with the traditional approach of conceptual analysis, the first stage will revue past uses of genealogical reasoning in relation to semantic externalism, tracing the development of genealogy from Hume through Darwin, Nietzsche, Wittgenstein and Foucault to E. J. Craig and B. Williams. A systematic genealogical approach to concepts will then be developed which innovatively differs from Foucault’s problematisations through historical derivation; it pursues instead a strategy indicated by Craig and Williams, which combines Nietzsche’s method with Hume’s fictional State of Nature stories and Wittgenstein’s understanding of concepts as tools. The resulting conception of genealogy will be refined and sharpened in a number of ground-breaking respects in a third stage, which will bring out the main problems and desiderata of this conception, namely (a) how the causal relations traced by genealogical explanations can account for and affect the rational relations in terms of which the current content of our concepts is articulated, and (b) how an answer to the first question can be used to clarify in what sense evolutionary, cultural or individual history determines content. In developing solutions to those problems, the project will issue in a novel two-level framework for a diachronic externalist approach to content, in which genealogies of concept-users’ understanding of concepts can be superimposed on genealogies of the concepts themselves. Finally, principled objections to the approach will be addressed and dissipated, in particular the charges of the genetic and the naturalistic fallacy.