Structure and Methodology: A Systematic Classification of World Religions and Sects

Al-Milal wan-Nihal — which translates as 'Religions and Sects' — is organized as a systematic survey of all the religious, philosophical, and theological traditions known to ash-Shahrastani. The title distinguishes between milal (religions, singular millah), understood as traditions with a revealed scripture or prophetic founder, and nihal (sects or philosophical schools, singular nihlah), understood as traditions based primarily on human reason and speculation. This distinction organizes the work's two-part structure.

The first and larger part covers the Islamic world: the theological sects of Islam (beginning with the Mutazilah, then the Shifis, then the Khawarij, and the Murji'ah, and the various Sifatiyyah schools including the Ash'ariyyah and Maturidiyyah), followed by pre-Islamic Semitic religions (Jews, Christians, Zoroastrians, and Sabians). The second part covers the philosophical and religious traditions of the non-Semitic world: the Greek philosophers from Thales through Plato and Aristotle, the Harranians, the Indian traditions, the Persians, and the pre-Islamic Arabian religious practices.

Ash-Shahrastani's methodological approach was genuinely innovative for his time. He aimed to describe each tradition as its adherents understood it — to present the strongest version of each school's arguments rather than a distorted or polemical caricature. This approach, which anticipates what modern scholars call the principle of charitable interpretation, was not universal in medieval heresiological literature. Many earlier Islamic works on sects and heresies were written to refute rather than to understand, and the difference between ash-Shahrastani's approach and theirs is significant.

He explicitly states in his introduction that he does not personally endorse the views he presents, that his goal is accurate description and classification rather than polemic, and that the reader should bear this in mind. This methodological disclaimer was important for establishing the scholarly legitimacy of his enterprise — studying other religions objectively was not self-evidently acceptable in a medieval Islamic context.

His use of earlier sources is extensive: for Greek philosophy, he draws on translated texts and the summaries available in Arabic; for Islamic theological schools, he draws on primary texts and the biographical and doxographical literature available to him. He is generally reliable on Islamic internal debates, somewhat less reliable on non-Islamic traditions where his sources were translations and summaries.