This work aims to cover a number of areas: giving an account of Leibniz's philosophy on individuals and the principle of individuation; providing an exploration of Leibniz's earliest philosophy, its sources in later scholastic philosophy, and the influence of those sources on his mature philosophy; and bringing together numerous Latin scholastic and Leibnizian texts, with many translated into English. On the basis of a study of Leibniz's earliest philosophy, an interpretation of central themes in Leibniz's philosophy is offered. This scholarly investigation demonstrates the persistence of premodern ideas in modern philosophy, calling into question in what sense Leibniz is a "modern" philosopher, and how the "modern" should be understood in modern philosophy and postmodernism. The intended audience includes historians of modern and later scholastic philosophy, Leibniz scholars, historians of ideas, philosophers with an interest in ontology and metaphysics, and graduate students in philosophy.