E.H. Palmer 
The mysticism of Islam [EPUB ebook] 

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 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.