Sources: Anan ben David, Book of Precepts (~770 AD); Saadia Gaon, Book of Beliefs and Opinions (~933 AD, the major rabbinic refutation); Judah Hadassi, Eshkol HaKofer (~1148 AD).

Karaism (from Mikra/Kara — Scripture) is the most significant internal Jewish heresy in the post-Talmudic period — a movement rejecting the entire Oral Torah and Talmud, insisting that the written Scripture alone is authoritative. Founded (in its classical form) by Anan ben David in Babylon (~770 AD), though with possible antecedents in earlier anti-rabbinic movements, Karaism spread rapidly through the Islamic world and reached its intellectual peak in the 10th–12th centuries. Key Karaite principles: (1) Sola scriptura for Judaism — every law must derive directly from the written Torah; (2) Individual interpretation — each person is obligated to study and interpret scripture for themselves; (3) Stricter practice in some areas — Karaites prohibit Sabbath fire (so no Sabbath lights), prohibit meat and dairy even separately (the rabbinic dairy/meat separation is seen as an extension beyond the text), and calculate the calendar by direct lunar observation rather than the rabbinic fixed calendar. The rabbinic response was fierce: Saadia Gaon’s comprehensive philosophical defense of oral tradition (Book of Beliefs and Opinions) is partly a sustained anti-Karaite polemic. At their peak, Karaites may have constituted 10–40% of world Jewry. Today approximately 30,000–40,000 Karaites survive, primarily in Israel.