Among Nietzsche's works there is a strange book which bears the title, Thus Spake Zarathustra. It consists of four parts, written during the years 1883-85, each part in about ten days, and conceived chapter by chapter on long walks--"with a feeling of inspiration, as though each sentence had been shouted in my ear," as Nietzsche wrote in a private letter.
The central figure and something of the form are borrowed from the Persian Avesta. Zarathustra is the mystical founder of a religion whom we usually call Zoroaster. His religion is the religion of purity; his wisdom is cheerful and dauntless, as that of one who laughed at his birth; his nature is light and flame. The eagle and the serpent, who share his mountain cave, the proudest and the wisest of beasts, are ancient Persian symbols.