Book excerpt: The tenth or eleventh century group of the Brethren of Purity (Ikhwan al Safa) are as well known in the Arab world as Darwin, Marx and Freud in the west. Designed as an introduction to their ideas, this book concentrates on the Brethren's writings, analyzing the impact on them of thinkers such as Pythagoras, Plato, Aristotle and the Neoplatonists. Ian Netton traces the influences of Judaism and Christianity, and controversially this book argues that the Brethren of Purity did not belong to the Ismaili branch of Islam as is generally believed.