Alain Badiou's 1983-1984 lecture series on "the One" is the earliest of his seminars that he has chosen to publish. It focuses on the philosophical concept of oneness in the works of Descartes, Plato, and Kant--a crucial foil for his signature metaphysical concept, the multiple. Badiou declares that there is no "One": there is no fundamental unit of being; being is inherently multiple. What is novel in Badiou's view of multiplicity is his reliance on mathematics, and set theory in particular. A set is a collection of things--yet, as he observes, it often is taken to "count as one" operationally for the purposes of mathematical transformations. In this seminar, distinguishing between "the One" and "counting as one" emerges as essential to Badiou's ontological project. His analysis of reflections on oneness in Descartes, Plato, and Kant prefigures core arguments of his defining work, Being and Event. Showcasing the seeds of Badiou's key ideas and later thought, The One features singular readings, breathtaking theorizations, and frequently astonishing offhand remarks.