This dissertation studies the problem of the inhuman in relation to human nature in philosophy from antiquity to the present, highlighting the interrelationship between science and philosophy in the development of concepts of monstrosity in France from mid-sixteenth century to late twentieth century thought. By means of constraint, it focuses on Ambroise Paré (1509/10–90) and Gilles Deleuze (1925–95) as representatives of early humanism and posthumanism, respectively. The study is divided into four chronologically ordered parts. In part I, four teratological traditions of philosophical import are discerned in antiquity: the naturalist, the humanist, the metaphysical, and the hermeneutical (each associated with a set of key names: in particular, Empedocles, Lucretius; Socrates, Protagoras; Plato, Aristotle; and Pliny, Augustine). Part II follows these traditions into the Renaissance where they intersect in the ‘books of wonder’, among which Paré’s Des monstres et prodiges (1573) is viewed to have had a lasting influence on the development of the science of teratology. Criticizing the positivistic conventions of interpretation of the book in question, notions of order, causality, diversity, and novelty are analyzed for the purpose of excavating from Paré’s work a natural philosophy which hinges on man’s capacity for knowledge; in such a humanist conception, monsters are not so much naturalized as nature becomes monstrous, while man is taken to reflect and encompass all the properties of natural things, thereby incorporating monstrosity in his singular variability. Part III provides an overview of the development of a scientific teratology from Cartesian mechanicism and rationalism, through theories of preformation, epigenesis, and transformation, to the materialist and vitalist debates of the early nineteenth century, when Étienne and Isidore Geoffroy Saint-Hilaire create the discipline of teratology, and its aftermath in developmental and evolutionary biology. The general theme is the place of anomalies in the normal scheme of nature (and culture), as man becomes progressively taken as the norm for thought, ultimately rendering the inhuman as such unthought. Finally, part IV looks to Deleuze as an attempt in the late 1900s to construct a posthumanist philosophy of nature where monstrosity is the problem which rather generates thought; it thus chronologically traces formulations of a concept of monstrosity in his body of work, from the 1940s to the 1990s. In Différence et répétition (1968), Deleuze is found to furnish three interconnected theses to define monstrosity, regarding problems of determination, synthesis, and differentiation, where the problematic as such (the nature of difference itself) is conceptualized as the ‘idea’ of monstrosity, not any particular physical shape. After analyzing the concept of the ‘body without organs’ as an issue of identity and materiality, tracing it back to its formulation in Logique du sens (1969), these theses of monstrosity are then applied to a study of Deleuze’s later philosophy, emphasizing Mille plateaux (1980), Logique de la sensation (1981), and Cinéma 1–2 (1983–85), as side-stepping the human norm in order to think its anomaly (the inhuman) as the condition for creativity. This is evidenced in his ideas of technology and the arts as experimental practices of becoming inhuman. The monster is thus regarded as a ‘conceptual persona’ in a Deleuzian philosophy of the virtual Figure—challenging all actual forms—of an inhuman time for the experience of difference in itself.