After submitting it as his doctoral dissertation he was awarded a PhD from the University of Jena in absentia. Private publication soon followed. "There were three reviews of it, commending it condescendingly. Scarcely more than one hundred copies were sold, the rest was remaindered and, a few years later, pulped." Among the reasons for the cold reception of this original version is a) it lacked the author's later authoritative style and b) appeared decidedly unclear in its implications. A copy was sent to Goethe who responded by inviting the author to his home on a regular basis, ostensibly to discuss philosophy but in reality to recruit the young philosopher into work on his theory of colours.
In 1847 Schopenhauer rewrote and enlarged the work, publishing a new edition. This is the version of the work that is read today. "There the lines of thought are firmly pursued, linking up with his main work; there a challenge is issued to philosophical tradition, and there is no curb on attacks against the philosophical spirit of the age."
In Schopenhauer’s point of view, Kant’s chief merit lies in his distinction between the thing in itself and the phenomenal world in which it appears, i.e., the world as we represent it to ourselves. What is crucial here is the realization that what makes experience possible to begin with and without exception is our perceiving mind, which synthesizes perceptions from raw sensation and consequently abstracts concepts from those perceptions. Schopenhauer appropriates Kant’s forms of sensibility (space, time, and causality) and transforms them into what he calls the understanding:
"To know causality is the sole function of the understanding, its only power, and it is a great power embracing much, manifold in its application, and yet unmistakable in its identity throughout all its manifestations. Conversely, all causality, hence all matter, and consequently the whole of reality, is only for the understanding, through the understanding, in the understanding. The first, simplest, ever-present manifestation of understanding is perception of the actual world. This is in every way knowledge of the cause from the effect, and therefore all perception is intellectual" (The World as Will and Representation volume I 11).
Thus, the understanding does not exist independent of our ability to perceive and determine relationships as it is the very ground of experience itself. Not only what we think in the abstract, but also our very perceptions are completely intellectual and subjectively determined. Already we have the philosophical grounds for Nietzsche’s perspectivism, though given in different language: representation (Vorstellung).
Schopenhauer’s central proposition, then, the main idea of his entire philosophy, he states simply as “The world is my representation.” The rest of his work is an elaborate analysis and unpacking of this sentence, which begins with his Kantian epistemology, but finds thorough elaboration within his version of the principle of sufficient reason. This is responsible for providing adequate explanations for any ‘thing,’ or object that occurs in relation to a subject of knowing; of any representation possible there is always a possible question of 'why?' that one can address to it. It amounts to what Schopenhauer has done, in his view, to extend and complete what Kant began with his Critique of Pure Reason.
Four classes of explanation fall under the principle’s rubric. Hence, four classes of objects occur always and already only in relation to a knowing subject, according to a correlative capacity within the subject. These classes are summarized as follows:
Different rules govern the possible explanations for representations of the four classes and “every explanation given in accordance with this guiding line is merely relative. It [the principle of sufficient reason] explains things in reference to one another, but it always leaves unexplained something that it presupposes,” and the two things that are absolutely inexplicable are the principle itself and the “thing in itself” (The World as Will and Representation volume I 81), which Schopenhauer connects with the will to live. The principle, in another point of view, provides the general form of any given perspective, presupposing both subject and object. The thing in itself, consequently, remains forever unknowable from any standpoint, for any qualities attributed to it are merely perceived, i.e., constructed in the mind from sensations given in time and space. Furthermore, because the concepts we form from our perceptions cannot in any way refer with any validity to anything beyond these limits to experience, all proofs for the existence of God or anything beyond the possibility of experience fall away under the razor of Kant’s critique. Kant termed this critical or transcendental idealism. Important to note here is that “Transcendental” does not refer to knowing the unknowable, but rather it refers to the a priori intellectual conditions for experience. This intuition of the a priori understanding is a modern elucidation of the postmodern expression "always already": time and space always and already determine the possibilities of experience. Additionally, Schopenhauer distinguishes from this something he calls a "spurious a priori": cultural perspectives (ideologies) one is born into that determine one's relationship to experience, in addition to the forms of space and time (Parerga and Paralipomena II 15). He considers these false because it is possible to investigate and uncover their grounds, leading to a reorientation that regards the phenomena of experience as source material of new knowledge, rather than one's always already prejudices about phenomena.