I aim to offer an innovative interpretation of Leibniz’s philosophy, first by examining how the various views that make up his ontology of individual substance involve a persistent rejection of atomism in natural philosophy and secondly, by exploring the significance of this rejection in the larger context of Seventeenth-century physics. My thesis is structured as a developmental story, each chapter analyzing the discontinuities or changes Leibniz makes to his views on individuation and atomism from his early to late years. The (...) goal is to illuminate underrepresented views on individuals and atoms throughout Leibniz’s works and thus bring a clearer understanding of his philosophy. I, therefore, argue that the New System of Nature, published towards the end of Leibniz’s middle period (1695), marks an important landmark in his philosophical evolution, a radical terminological and ontological shift in his metaphysics of substance. Once Leibniz elaborates the concept of “simple substance,” the future synonym of “monad,” the problem of individuation of his early and middle years (1663-1686) becomes secondary. The focus changes from what makes substances “individual” to what makes them “simple” and truly “one,” i.e., “metaphysical” atoms. I prove that this shift was marked by a two-tiered critical confrontation: a first, direct confrontation, 1) with Descartes’ physics, through the critique of the notion of extended matter and of Descartes’ principle of individuation through shared motion and, a second confrontation, 2) with different strands of Seventeenth-century atomism, including Cartesian Gérauld de Cordemoy’s quasi-“metaphysical” atomism and its attempt at improving Descartes’ individuating principle. I claim that this double confrontation ultimately led Leibniz to formulate a more fundamental ontology, in terms of the “metaphysical atomism” of his Monadology (1714). My analysis complicates a persistent scholarly assumption in recent Leibniz studies, claiming that, throughout his entire career, Leibniz continued to hold the same fundamental positions on substance, individuation and, implicitly, atoms. Against this type of general continuity thesis, I show that: 1) far from being a constant concern, Leibniz’s interest in what makes substances individual fades towards the end of his life (New Essays 1703, correspondence with Samuel Clarke, 1714); 2) I trace the changing fate of some of Leibniz’s early and middle period views on substance and the individual (the principle of the identity of indiscernibles, space-time as individuating properties) in his late works; and 3) I prove the claim that Leibniz really embraced atomism, either for a short time or all throughout his philosophy is problematic. While he does refer to some sort of atoms during his Paris period (1672-1676), this is insufficient proof of a commitment to atomism. Instead, the episode has to be understood in the broader framework of a bundle of interrelated issues, such as the problem of the cohesion of bodies and the problem of minds or mind-like principles individuating those bodies. Thus, as I show through an analysis of Leibniz’s arguments against atomism in the correspondences with his scientific contemporaries (Christiaan Huyghens 1692-1695, Nicholas Hartsoeker 1706-1714), rejecting physical atomism remains a fundamental and surprisingly constant point of his philosophy. (shrink)