This volume collects 12 essays by various contributors on the subject of the importance and influence of Schopenhauer's doctoral dissertation (On the Fourfold Root of the Principle of Sufficient Reason) for both Schopenhauer's more well-known philosophy and the ongoing discussion of the subject of the principle of sufficient reason. The contributions deal with the historical context of Schopenhauer's reflections, their relationship to (transcendental) idealism, the insights they hold for Schopenhauer's views of consciousness and sensation, and how they illuminate Schopenhauer's theory of action. This is the first full-length, English volume on Schopenhauer's Fourfold Root and its relevance for Schopenhauer's philosophy. The thought-provoking essays collected in this volume will undoubtedly enrich the burgeoning field of Schopenhauer-studies.