This work provides an overview of the contributions as a thinker of Abu Ya'qub al-Sijistani, a 10th-century missionary whose writings reveal him as both a philosopher and an exponent of the intellectual understanding of Islam. The old problem of the meaning of science and religion and their interaction as reflected in the thought of an Ismaili author from the early Islamic period is now interpreted within the framework that brings together ideas and obscure doctrines surviving only piecemeal from medieval Arabic books and treatises.