Sources: Saadia Gaon, Sefer Emunot ve-De’ot (Book of Beliefs and Opinions, ~933 AD); Maimonides, Guide for the Perplexed (~1190 AD); Judah Halevi, Kuzari (~1140 AD).

Saadia ben Joseph al-Fayyumi (882–942 AD), the Gaon of the Sura Academy, is the founder of systematic Jewish philosophy — the first thinker to engage Islamic Kalam (rationalist theology) on its own terms and apply it to Jewish doctrine. His Sefer Emunot ve-De’ot is the first systematic theological work in Jewish literature, written in Judeo-Arabic and later translated to Hebrew. Saadia’s central question: what is the relationship between emunah (faith, religious tradition) and da’at (knowledge, rational inquiry)? His answer: there is no conflict. All truths accessible to reason are also accessible through prophetic revelation — because both have the same divine source. Where they appear to conflict, either the rational argument has an error or the traditional text requires non-literal interpretation. Four sources of knowledge: sense perception, reason, logical inference, and reliable tradition. For Jews, scripture and rabbinic tradition constitute reliable tradition — but they must be consistent with what reason establishes. This rationalist framework defends traditional Judaism against Karaites (who rejected oral Torah) and against Islamic and Christian challenges to Jewish doctrine. Saadia’s method — using philosophical argument to defend religious tradition — defines the entire medieval Jewish philosophical tradition from Bahya ibn Paquda through Maimonides and Gersonides, while his critics (Judah Halevi) argued that the divine revelation to Israel is precisely what cannot be captured by universally accessible reason.