 Schopenhauer puts forward his theory in his main work The World as Will and Representation (Die Welt als Wille und Vorstellung), first published in December, 1818 (with an 1819 date on its title page), and then in a much-expanded second edition in 1844 and yet another expanded edition in 1857. This book had been preceded by a doctoral dissertation On the Fourfold Root of the Principle of Sufficient Reason (1813), which Schopenhauer subsequently regarded as the introduction to his magnum opus. The earlier work includes Schopenhauer’s main modifications to the structure of Kant’s epistemology, while the later work accepts Kant’s idealist interpretation of this epistemology (Book I) and then replaces Kant’s version of the doctrine of things in themselves with Schopenhauer’s own version of the unitary non-rational will underlying all appearance (Book II).