Arthur Schopenhauer never characterized his philosophy or himself as “pessimistic,” though he never rejected the characterization when it was applied to him. He uses the word only four times in The World as Will and Representation, according to the comprehensive index of both volumes. Neither the words “optimistic” or “pessimistic” can be readily applied to the tragic consciousness — there is a determinism in both words that stifles a more nuanced conception of the form and the consciousness — but Schopenhauer is ready to explain why optimism itself is unwarranted, even destructive. His most pointed comment on this below:

At bottom, optimism is the unwarranted self-praise of the real author of the world, namely of the will-to-live which complacently mirrors itself in its work. Accordingly optimism is not only a false but also a pernicious doctrine, for it presents life as a desirable state and man’s happiness as its aim and object. Starting from this, everyone then believes he has the most legitimate claim to happiness and enjoyment. If, as usually happens, these do not fall to his lot, he believes that he suffers an injustice, in fact that he misses the whole point of his existence; whereas it is far more correct to regard work, privation, misery, and suffering, crowned by death, as the aim and object of our life … since it is these that lead to the denial of the will-to-live. In the New Testament, the world is presented as a vale of tears, life as a process of purification, and the symbol of Christianity is an instrument of torture. Therefore, when Leibniz, Shaftesbury, Bolingbroke, and Pope appeared with optimism, the general offence caused by it was due mainly to the fact that optimism is irreconcilable with Christianity. This is stated and explained by Voltaire in the preface to his excellent poem Le Désastre de Lisbonne, which also is expressly directed against optimism. This great man, whom I so gladly commend in the face of the slanders of mercenary German ink-slingers, is placed decidedly higher than Rousseau by the insight to which he attained in three respects, and which testifies to the greater depth of his thinking: (1) insight into the preponderating magnitude of the evil and misery of existence with which he is deeply penetrated; (2) insight into the strict necessitation of the acts of will; (3) insight into the truth of Locke’s principle that what thinks may possibly be also material. … Rousseau’s whole philosophy is that he puts in the place of the Christian doctrine of original sin and of the original depravity of the human race an original goodness and unlimited perfectibility thereof, which had been led astray merely by civilization and its conequences; and on this he then establishes his optimism and humanism.

Arthur Schopenhauer
“On the Vanity and Suffering of Life”
The World as Will and Representation, Vol. II (584-585)
Translated by E.F.J. Payne

Schopenhauer’s thoughts are echoed by American novelist William Gaddis in discussing Wyatt Gwyon, the main character of his masterpiece The Recognitions. In a 1960 letter, Gaddis wrote:

Wyatt has arrived at a doctrine somewhat akin to Gide’s ― a doctrine which holds that salvation lies in scraping away the consolatory deceits and secondhand values of the counterfeit personality and in obeying the promptings of the real self, the soul, in the full awareness that man is “born into sin” and that sin must be “lived through”: all efforts to escape from the burden of imperfection are a denial of humanity and therefore lead to spiritual and emotional forgery.

The full letter will appear in The Letters of William Gaddis, edited by Stephen Moore and scheduled for publication by the Dalkey Archive Press in February 2013.