The following work is founded upon a Persian MS. treatise by 'Aziz bin Mohammed Nafasi, but I have endeavoured to give a clearer and more succinct account of the system than would have been afforded by a mere translation. The term Sufi is derived from the Arabic word suf "wool," in allusion to the dress adopted by the Dervishes, who are the master and teachers of the sect; the similarity to the Greek sofos appears to be merely accidental. The system of the Sufis consists in endeavouring to reconcile Philosophy with Revealed Religion, and in assigning a mystical and allegorical interpretation to all religious doctrines and precepts. These tenets are found principally among the Shi'ites, or followers of 'Ali, and appear to have existed in Islamism from its very foundation; indeed the expression of the Coran, "I am the Truth" (Hacc), is the first principle of the system. They may be considered as forming the esoteric doctrine of that creed. Steering a mid course between the pantheism of India on the one hand and the deism of the Coran on the other, the Sufis' cult is the religion of beauty, where heavenly perfection is considered under the imperfect type of earthly loveliness. Their principal writers are the lyric poets, whose aim is to elevate mankind to the contemplation of spiritual things, through the medium of their most impressionable feelings. This habit of contemplation, which is so constantly inculcated by them, requiring as it does retirement and seclusion for its due exercise, inclines the followers of the system somewhat towards asceticism, but in countries where luxury is the idol of the many, we may not unnaturally look for a protest against it in the tendencies of the few. My present intention is merely to give an exposition of the system; its origin and history I reserve for a future work, in which I hope to prove that Sufiism is really the development of the Primaeval Religion of the Aryan race. The Ahl i wahdat form a branch of Sufiism, rather than a separate sect of Theosophists; they insist upon the Universality and Unity of God. I have translated the title "Unitarian," although I am sensible that misapprehension may arise in consequence of its current application to the professors of a particular form of modern belief. I should have preferred the use of some such term as Monopantachists had I possessed sufficient courage or position to warrant me in coining so formidable an epithet. The term may be generally understood of those Mussulmans, who, though pursuing philosophical enquiry, refuse to subscribe unreservedly to all the metaphysical doctrines of the Sufis. The expression zat i Khuda, "the Nature of God," by which the Persians designate the very essence and being of the Deity, would, perhaps (according to the general use of the word zat in construction with a proper name), be more idiomatically rendered "God Himself;" but as this treatise professes to deal in exactitudes investigated from an Oriental point of view, I have preferred keeping to the original idiom as more definitely expressing the idea. In conclusion, I have only to acknowledge my obligation to Mr. C. A. Hope, of St John's College, for his valuable assistance afforded me in preparing this book for the press.