outside, an expression with which Schopenhauer often expressed his dissatisfaction. He, on the other hand, was occupied with the questions: how do we get this empirical content of perception; how is it possible to comprehend subjective sensations "limited to my skin" as the objective perception of things that lie "outside" of me? Causality is therefore not an empirical concept drawn from objective perceptions, as Hume had maintained; instead, as Kant had said, objective perception presupposes knowledge of causality. By this intellectual operation, comprehending every effect in our sensory organs as having an external cause, the external world arises. With vision, finding the cause is essentially simplified due to light acting in straight lines. We are seldom conscious of the process that interprets the double sensation in both eyes as coming from one object, that inverts the impressions on the retinas, and that uses the change in the apparent position of an object relative to more distant objects provided by binocular vision to perceive depth and distance. Schopenhauer stresses the importance of the intellectual nature of perception; the senses furnish the raw material by which the intellect produces the world as representation. He set out his theory of perception for the first time in On Vision and Colors, and, in the subsequent editions of Fourfold Root, an extensive exposition is given in § 21. The world as will In Book Two of The World as Will and Representation, Schopenhauer considers what the world is beyond the aspect of it that appears to us—that is, the aspect of the world beyond representation, the world considered "in-itself" or "noumena", its inner essence. The very being in-itself of all things, Schopenhauer argues, is will (Wille). The empirical world that appears to us as representation has plurality and is ordered in a spatio-temporal framework. The world as thing in-itself must exist outside the subjective forms of space and time. Although the world manifests itself to our experience as a multiplicity of objects (the "objectivation" of the will), each element of this multiplicity has the same blind essence striving towards existence and life. Human rationality is merely a secondary phenomenon that does not distinguish humanity from the rest of nature at the fundamental, essential level. The advanced cognitive abilities of human beings, Schopenhauer argues, serve the ends of willing—an illogical, directionless, ceaseless striving that condemns the human individual to a life of suffering unredeemed by any