On the Fourfold Root of the Principle of Sufficient Reason is an elaboration on the classical principle of sufficient reason, written by German philosopher Arthur Schopenhauer as his doctoral dissertation in 1813. The principle of sufficient reason is a powerful and controversial philosophical principle stipulating that everything must have a reason or cause. Schopenhauer revised and re-published it in 1847. The work articulated the centerpiece of many of Schopenhauer's arguments, and throughout his later works he consistently refers his readers to it as the necessary beginning point for a full understanding of his further writings.

Arthur Schopenhauer was a German philosopher. He is known for his 1818 work The World as Will and Representation, which characterizes the phenomenal world as the manifestation of a blind and irrational noumenal will. Building on the transcendental idealism of Immanuel Kant (1724–1804), Schopenhauer developed an atheistic metaphysical and ethical system that rejected the contemporaneous ideas of German idealism.