Schopenhauer read the Oupnek'hat, which is a collection of Upanishads, mixed with the commentary of Shankara, which the freethinking sultan Dara Shikoh had ordered to be translated to Persian. According to the scholar Paul Deussen, it is an extremely bothersome work to read:

>Owing to the excessive literality with which Anquetil Duperron rendered these Upanishads word by word from the Persian into Latin, while preserving the syntax of the former language, — a literality that stands in striking contrast to the freedom with which the Persian translators treated the Sanskrit text, — the Oupnek'hat is a very difficult book to read ; and an insight as keen as that of Schopenhauer was required in order to discover within this repellent husk a kernel of invaluable philosophical significance, and to turn it to account for his own system. (The Philosophy of the Upanishads)