The title of this book sufficiently explains why it is included in a Series 'exemplifying the adventures and labours of individual seekers or groups of seekers in quest of reality.' Sufism, the religious philosophy of Islam, is described in the oldest extant definition as 'the apprehension of divine realities,' and Mohammedan mystics are fond of calling themselves Ahl al-Haqq, 'the followers of the Real.'In attempting to set forth their central doctrines from this point of view, I shall draw to some extent on materials which I have collected during the last twenty years for a general history of Islamic mysticism-a subject so vast and many-sided that several large volumes would be required to do it anything like justice. Here I can only sketch in broad outline certain principles, methods, and characteristic features of the inner life as it has been lived by Moslems of every class and condition from the eighth century of our era to the present day. Difficult are the paths which they threaded, dark and bewildering the pathless heights beyond; but even if we may not hope to accompany the travellers to their journey's end, any information that we have gathered concerning their religious environment and spiritual history will help us to understand the strange experiences of which they write.