In the Appendix, Schopenauer wrote that "next to the impression of the world of perception, I owe what is best in my own development to the impression made by Kant's works, the sacred writings of the Hindus, and Plato" (WWR I, p. 417). Before looking at the philosophical works just mentioned, I will say a bit about the influence of the world of perception on Schopenhauer.
Schopenhauer found in the world of perception a spectacle of incessant change, which results from the activities of relentless forces. The procession of the heavenly bodies, the cascading of the waterfall, the howling of the wind, the pursuit of food and shelter, the sexual drive -- all bespeak of an underlying striving which Schopenhauer called "will."
Human activity is no exception. The only difference is that human efforts are guided by motives, by abstractly conceived ends toward which our efforts are pointed. It was Schopenhauer's observation that the dominant end of most people's actions is themselves. "The chief and fundamental incentive in man as in the animal is egoism, that is, the craving for existence and well-being. . . . By its nature, egoism is boundless; man has the unqualified desire to preserve his existence, to keep it absolutely free from pain and suffering, which includes all want and privation. He desires to have the greatest amount of well-being and every pleasure of which he is capable . . . " (On the Basis of Morality, tr. E. F. J. Payne, The Library of Liberal Arts, 1965, p. 131). This conclusion is based on Schopenhauer's impressions of the world of perception, filtered to some extent by the English newspapers of the day, which he read assiduously. His ethical theory condemns egoism as anti-moral.
While other philosophers have so marveled at the workings of the world that they attribute them to a wise creator and architect (God), Schopenhauer was appalled at the misery and general imperfection of the world. He even adavanced an argument to the conclusion that our world is the worst possible world, because it could not become worse without ceasing to exist altogether. In an example which seems remarkably prescient, he wrote, "A very moderate increase of heat would dry up all the rivers and springs" (WWR II, 583). And little has changed since he wrote, "Powerful as are the weapons of understanding and reason possessed by the human race, nine-tenths of mankind live in constant conflict with want, always balancing themselves with difficulty and effort on the brink of destruction" (WWR II, 584). Scohpenhauer noted that the great writers and religions of all ages have expressed the wretchedness of existence.
Schopenhauer's closest companions were a series of poodles. He remarked that animals are more likeable than people because they are incapable of deception. Indeed, Schopenhauer was one of the earliest advocates of the rights of animals. On the other hand, due no doubt in part to his conflicts with his mother, Schopenhauer had a low regard for women many respects. His essay "On Women," is one of the most notorious works ever written on the subject.
Let us turn now to the strictly philosophical influences mentioned above. We begin with the most ancient philosophy, the Hindu. Schopenhauer was acquainted with Indian philosophy through the works of German scholars who provided the first translations of the texts into a European language. Most important for Schopenauer is the doctrine of Maya, according to which the world of perception is illusory. "The work of Maya is stated to be precisely this visible world in which we are, a magic effect called into being, an unstable and inconstant illusion without substance, comparable to the optical illusion and the dream, a veil enveloping human consciousness, a something of which it is equally false and equally true to say that it is and that it is not" (WWR I, p. 419).
The same theme is to be found in the ancient philosophy of the Greeks. Parmenides, a predecessor of Plato, had declared that according to the way of truth, reality is one and unchangeable. Plurality and change are consigned to the way of opinion. On this view, it is impossible to explain the world of perception by reference to the real. Plato, by contrast, held that an explanation could be given by allowing a plurality of real unchangeable forms. Thus there can be many things of one kind insofar as each "partakes" of a single form. A thing changes, becomes of another kind, insofar as it partakes of a different form. But there is still a missing element here, i.e. an explanation of the agent of change, that which is responsible of the shift from form to form.
The forms are realities and the world of perception is no more than their shadows. Schopenhauer took Plato's cave analogy to express this fundamental thesis. (Plato's philosophy, like that of the Hindus, he held to be more allegorical than philosophical.) Our relation to objects of perception is like that of someone chained in a cave, able to view only shadows of real things behind them. Although Schopenhauer identified the fundamental reality with will, he made a place for "Platonic forms" in his system. We approach these forms through contemplation of art, which provides temporary relief from our miserable existence.
It remains to fill in the gap between Plato and Kant. Aristotle had claimed that the ultimate agent of change is a divine intellect. Christian thinkers expanded on this theme. The Platonic forms, that which gives structure to the perceptual world, are replaced by conceptions in God's understanding, and change is explained by the agency of God's will.
The rise of modern science posed a threat to this mode of explanation. Structure could be found in mathematical laws of nature, and activity could be explained by reference to physical forces. Thus science posed the threat of materialism, and most philosophers of the seventeenth and eighteenth centuries recoiled against this threat, proposing various schemes for reconciling science and religion.
One of those philosophers was René Descartes. He maintained that the human faculties for investigating the world of perception are inadequate in and of themselves. Any evidence from the senses could be doubted, and even the use of reason to correct the senses is not justified without God's guarantee that reason does not lead us astray. The only certainty which we can attain by ourselves is self-certainty. I can be certain that I exist and of what I am. Descartes had made subjectivity the starting point of philosophy, and according to Schopenhauer, "It is thus rightly considered that the philosophy of the moderns starts from Descartes as its father" (WWR II, p. 4).
But it was George Berkeley who gave philosophy a more radically subjective turn, "and arrived at idealism proper; in other words, at the knowledge that what is extended in space, and hence, the objective, material world in general, exists as such simply and solely in our representation . . ." (WWR II, p. 4). This "very correct and deep insight" was adopted by Schopenhauer himself, under the slogan "the world [i.e., the world of perception] is my representation."
Berkeley's slogan for the existence of perceptual objects was that their being is to be perceived. Schopenhauer expressed it more generally as "no object without a subject." That is, it is impossible for something to be an object of human conception without its being conceived by a human subject. As Berkeley put it, any attempt to conceive of perceptual objects as existing "without the mind" requires "that you conceive them existing unconceived or unthought of, which is a manifest repugnancy. When we do our utmost to conceive the existence of external bodies, we are all the whole contemplating our own ideas" (Principles of Human Knowledge, Section 23).
Deep as Berkeley's insight was, it concerned only the general fact about the status of the world, and did not explain how the world exists in our representation. This task, according to Schopenhauer, was completed by Immanuel Kant. There is much contention about this point, however. In construing Kant as taking Berkeley's idealism one step further, Schopenhauer's interpretation ran head-on into Kant's own disavowal of Berkeley's idealism in the second edition (and subsequent editions) of the Critique of Pure Reason. Schopenhauer's response was to accuse Kant of contradicting his own doctrine.
It was not until later in his career that Schopenhauer discovered the neglected first edition of the Critique. Here, he thought he had found a consistent doctrine which accorded perfectly with Berkeley's idealism. The ever-suspicious Schopenhauer explained the later disavowal of Berkeley as the result of Kant's having caved in to criticism of his embrace of Berkeley's idealism.
Let us now look in more detail at the doctrine Kant himself called "transcendental," "formal," or "critical" idealism. Kant described the world of perception as "appearance" or "phenomena," which are contrasted to "things in themselves." Schopenhauer interpreted "things in themselves" as the reality lying behind the appearances, a reality which he himself would interpret as "will."
Kant held that we represent the world of perception in two distinct ways: perceptually and intellectually. We perceive the world as consisting of objects in space and time. Kant held that space and time are forms which are contributed by the human faculty of sensible or perceptual representation. If we try to consider things in abstraction from space and time, we come up empty, since all the properties we attribute to perceptual objects depend in some way on space and time.
Human intellect or understanding represents objects through general concepts. Kant maintained that there are twelve fundamental concepts (categories) contributed by the human understanding in its representation of objects of perception. Schopenhauer reduced the list to one: causality. Thus the causal relations of perceptual objects to one another are the work of representation and cannot be found in things in themselves. The causal relations are the source of laws of nature, which therefore are subjective in origin. In the first edition of the Critique, Kant stated the point bluntly: "However exaggerated and absurd it may sound, to say that the understanding is itself the source of the laws of nature, . . . such an assertion is none the less correct." (Transcendental Deduction, Section 3).