The influence of Al-Ghazzali upon both the Christian and Islamic thinkers of the Middle Ages and beyond is being more and more widely documented. Known as "The Proof of Islam." Ghazzali finally won acceptance for Sufism in Islam, and his methods of argument and analysis powerfully impressed the schoolmen of the West, who imitated him extensively. In the East, "Al-Ghazzali has been acclaimed by both Muslim and European scholars as the greatest Muslim after Muhammad" (Professor W. Montgomery Watt). Above all, Ghazzali was a Sufi, and The Alchemy of Happiness is his own abridgement, designed for the ordinary reader, of his colossal masterwork, The Revival of Religious Sciences.