This thesis focuses on the trajectory of the biomedical concept of virulence from 1880 until the present. Following the concept across disciplinary boundaries, from a longue durée history perspective, it explores how virulence was shaped through two distinct, although sometimes overlapping, “styles of reasoning”. Located at the intersection of several distinct research domains in biology and medicine, the concept of virulence provides, in addition, a window into the complex and changing relations between evolutionary biology and the health sciences (broadly construed) over the past two centuries. Moving back and forth between field experiments and the laboratory, this work examines, through the lens of historical epistemology, the emergence of what I call the molecular and the ecological styles, and their respective conceptual practices. It focuses on the ways in which these styles operationalize the distinction between virulent or avirulent organisms in sometimes opposite sense: Whereas in the molecular (or endogenous) style the expression of virulence is explained by properties of internal structures of the infectious agent (e.g. polysaccharide capsule, virulence gene, or pathogenicity island), the concept of virulence in the ecological (or exogenous) style reflects, in contrast, either a lack of adaptation between two species (avirulence hypothesis) or the existence of one or more ecological compromises between, say, the mode of transmission of a pathogen and its host’s recovery rate (trade-off model). Both styles can be said to originate in the medical bacteriology of the late-nineteenth century, but while the former grew mostly out of the work of Louis Pasteur and Robert Koch in Europe, the latter was primarily shaped by Theobald Smith in the United States. Nearly a century later, the introduction of the category of emerging infectious disease within public health discourses in the mid-1990s facilitated a rapprochement between the two styles that had, so far, remained apart. Employing the 1918–1919 influenza pandemic as an example in which to illustrate the trajectory of the molecular and the ecological approaches, the diversity of explanatory schemes developed to account for the pandemic’s exceptional virulence points toward an unresolved, and yet productive, epistemic tension between the two styles, on the one hand, and the intrinsic polarity of the concept of virulence itself, on the other.

Ma recherche a pour but de faire le point sur les derniers développements de l’historical epistemology, modalité d’enquête de la connaissance qui a émergée au cours du XX siècle au sein de l’épistémologie française et qui reste actuellement active dans une variété de formes. En dépit de son succès auprès de nombreux auteurs contemporains, l’épistémologie historique ne bénéficie encore d’aucune systématisation ni d’aucune liste d’auteurs et d’ouvrages canoniques. Mon travail de thèse se propose de traiter directement la question même de la légitimité et de l’originalité de ce type d’épistémologie en discutant sa nature, sa méthodologie et son unité. En analysant les ouvrages les plus importants dans ce domaine, j’accorderai un rôle central d’un coté à Michel Foucault et de l’autre à Ian Hacking, qui, à maintes égards, ont entretenu des rapports complexes, controversés, et pourtant révélateurs, avec l’épistémologie historique. Les deux phases de l’épistémologie historique, l’une « originaire » et l’autre « contemporaine », seront analysées dans un rapport biunivoque, dans un souci de clarification réciproque. Le développement à l’étranger de l’épistémologie historique dans sa phase contemporaine a créé une sorte de vide et une perte d’intérêt étonnante dans son lieu de naissance. La reconnexion de ce type d’épistémologie au cadre philosophique originel de son émergence représente l’occasion de ré-ouvrir le débat en France. / What is historical epistemology? Why does this field, despite its current proliferation,seem to be permanently haunted by questions relative to its nature, limits and ultimatetasks? What kind of historicization is at stake in this sort of inquiry? What is the relationbetween contemporary historical epistemology, as it is practiced by a growing number ofEnglish-speaking historians and philosophers of science, and the French “tradition” ofépistémologie historique? To address these questions, my research aims to provide arecursive analysis demonstrating how the two phases of historical epistemology, the“classical” and the “contemporary”, can clarify each other. In this process, the“archaeological method” of Michel Foucault, which draws on and transforms fundamentalinsights by Gaston Bachelard and Georges Canguilhem, will be shown to exert an enduringinfluence on the field, especially through Ian Hacking and his philosophical cum historicalanalyses of probability. / Che cos’è l’epistemologia storica? Perchè questo campo, nonostante la sua proliferazione attuale, sembra essere permanentemente minacciato da questioni relative alla sua natura, ai suoi limiti e ai suoi obiettivi ultimi? Che tipo di storicizzazione caratterizza questo tipo di indagine? Qual è la relazione tra l’epistemologia storica contemporanea, come è praticata da un numero crescente di storici e filosofi della scienza di lingua inglese, e la tradizione francese dell’épistémologie historique? Per affrontare tali questioni, la mia ricerca intende fornire un’analisi ricorsiva che dimostri come le due fasi dell’epistemologia storica, quella “classica” e quella “contemporanea”, possono chiarificarsi reciprocamente. In questo processo, il “metodo archeologico” di Foucault, che trae spunto da e trasforma intuizioni fondamentali di Gaston Bachelard e Georges Canguilhem, sarà mostrato nella sua influenza su questo campo di indagine, specialmente attraverso Ian Hacking e le sue analisi storico-filosofiche della probabilità.

History is descriptive. Epistemology is conceived as normative. It appears, then, that a historical approach to epistemology, like historical epistemology, might not be epistemically normative. In our context here, epistemology is not a systematic theory of knowledge, truth, or justification. In this thesis I approach epistemic justification through the vantage point of practice of science. Practice is about reasoning. Reasoning, conceived as the human propensity to order perceptions, beliefs, memories, etc., in ways that permit us to have understanding, is not only about thinking. Reasoning has to do with our actions, too: In the ordering of reasoning we take into account the desires of ourselves and others. Reasoning has to do with tinkering with stuff, physical or abstract. Practice is primarily about skills. Practices are not mere groping. They have a form. Performing according to a practice is an activity with a lot of plasticity. The skilled performer retains the form of the practice in many different situations. Finally, practices are not static in time. Practices develop. People try new things, some of which may work out, others not. The technology involved in how to go about doing things in a particular practice changes, and the concepts concerning understanding what one is doing also may change. This is the point where history enters the picture. In this thesis I explore the interactions between history, reasoning, and skills from the viewpoint of a particular type of epistemic justification: a priori justification. An a priori justified proposition is a proposition which is evident independent of experience. Such propositions are self-evident. We will make sense of a priori justification in a context of regarding science as practice, so that we will be able to demonstrate that the latter accommodates the normative character of science.