This text looks at the tenth-century missionary, al-Sijistani. Though long neglected by historians of Islamic philosophy, al-Sijistani's recently recovered writings proves that he deserves careful consideration both as a philosopher and as 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 a remote period, is now interpreted within a framework that provides broad coherence to disparate ideas and obscure doctrines which survive only piecemeal from medieval Arabic books and treatises. Here, al-Sijistani's contributions appear all the more cogent and impressive despite the 1000 years that separate him from us.