Léon Brunschvicg''s contribution to philosophical thought in fin-de-siècle France receives full explication in the first English-language study on his work. Arguing that Brunschvicg is crucial to understanding the philosophical schools which took root in 20th-century France Pietro Terzi locates Brunschvicg alongside his contemporary Henri Bergson as well as the range of thinkers he taught and influenced including Lévinas Merleau-Ponty de Beauvoir and Sartre. Brunschvicg''s deep engagement with debates concerning spiritualism and rationalism neo-Kantian philosophy and the role of mathematics in philosophy made him the perfect supervisor for a whole host of nascent philosophical ideas which were forming in the work of his students. Terzi outlines Brunchvicg''s defence of neo-Kantian judgement historical analysis and the inextricability of the natural and humanist sciences to any rigorous system of philosophy with wide-ranging implications for contemporary scholarship.