This book, based upon a Jordan Lecture in Comparative Religion of 1959, traces the development of mystical thought during the formative periods of the Hindu and Muslim traditions. The religions are discussed separately but comparisons are offered wherever appropriate. The part on Hinduism focuses on the classical Upanishads, the Yogasutras, the Bhagavad-Gita, and Ramanuja's commentary on them. For Islam, the focus is on the monistic revolution introduced by Abu Yazid, which Zaehner traces to the influence of Indian thought and through Junayd's restoration of the theistic balance to the monism of the late writings of Ghazali.