Mysticism is such a vital element in Islam that without some understanding of its ideas and of the particular forms which they assume it is hard to penetrate below the surface of Muslim religious life. In this book, which was first published in 1921, Professor Nicholson examines the life, work and teaching of three of the most important of the early Sufis - the Persians Abu Sa'id (937-1049) and Al-Jili (1365-1406), and the Cairene Arab Ibnu l-Farid (1182-1235). These great mystics were almost legendary figures; possessors of occult and mysterious powers, whose tombs became holy shrines. They were regarded in effect as saints, but saints canonised by the people while still living, not posthumously by the church. Sufism, as Professor Nicholson suggests, lies at the heart both of the religious philosophy and the popular religion of Islam.