On the Principle of Sufficient Reason

"Schopenhauer’s analyses of causation and related concepts ... rival and probably surpass in their depth and brilliance the more celebrated discussions of David Hume. Where Hume grossly oversimplified these problems and left them riddled with paradoxes, Schopenhauer disentangled them and shed light on what had seemed hopelessly dark" – Richard Taylor
This little-known work by the famous German pessimist and critic of Hegel was originally written as a doctoral dissertation when Schopenhauer was just twenty-six, but it was later revised when the philosopher was sixty. So important did he consider this work, originally titled "On the Fourfold Root of the Principle of Sufficient Reason," that he often underscored the fact that no one could hope to understand his magnum opus, The World as Will and Representation, without having first read this work. Schopenhauer takes up where Kant left off in response to Hume, and his insights into the nature of perception and understanding remain amazingly relevant and still unsurpassed.
"Our knowing consciousness, which manifests itself as outer and inner sensibility (or receptivity) and as understanding and reason, subdivides itself into subject and object and contains nothing else. To be object for the subject and to be our representation are the same thing. All our representations stand towards one another in a regulated connection, which may be determined a priori, and on account of which nothing existing separately and independently, nothing single or detached, can become an object for us. It is this connection which is expressed by the Principle of Sufficient Reason in its generality." – chapter 3, §16