The book opens with an investigation into the Principle of Sufficient Reason, wherein Schopenhauer argues that previous philosophers confused logical reasons with physical causes. He traces this intellectual error from Plato and Aristotle through Descartes and Spinoza up to Kant and his contemporaries. By clarifying these distinctions, the author lays the groundwork for a systematic critique of how human consciousness apprehends reality.
The inquiry establishes that the principle operates through four distinct roots corresponding to four classes of objects for the subject. The first class comprises intuitive, empirical representations governed by the law of causality, which rules changes in the material world. Schopenhauer demonstrates that empirical perception is fundamentally an intellectual process performed by the brain rather than a mere passive reception by the senses.
The second class of objects consists of abstract conceptions mediated by the faculty of Reason and language. Here, the principle of sufficient reason appears as the principle of knowing, dictating that every true judgment must rest upon a sufficient reason of logical, empirical, transcendental, or metalogical truth. This framework explicitly separates the conceptual operations of human Reason from the intuitive mechanics of the Understanding.
The third class encompasses pure intuitions of space and time, where the principle governs the reason of being. This section explores the foundations of arithmetic and geometry, distinguishing between the reason of being and the reason of knowing in mathematical demonstrations. The author contends that genuine insight requires grasping the inner reason of being rather than merely attaining logical conviction through formal proofs.
The fourth class is the immediate object of inner sense: the human will. Schopenhauer connects motivation to causality, presenting the action of motives as causality viewed from within. This subjective experience provides the crucial key to his broader metaphysical system.
Throughout the essays, the author maintains that all knowledge is fundamentally constrained by the subjective forms of the intellect. He critiques contemporary academic philosophy for abandoning Kantian rigor in favor of groundless assertions about supersensuous faculties. The volume concludes by reinforcing the strict limits of human cognition and the necessity of anchoring philosophical speculation in observable reality.
