Anan ben David ענן בן דוד

Precision matters here: Anan is usually called the founder of Karaism, but it is more accurate to call him the founder of the Ananites, a separate current out of which Karaism took shape later, in the ninth and tenth centuries — and not out of it alone.

The point of divergence is simple and enormous: whether to follow Scripture as it reads, or as the Oral Torah expounds it. Rabbinic Judaism answers the second; the Ananites and Karaites the first.

He is also credited with the slogan "search well in Scripture and do not rely on my opinion." The attribution is unreliable: the first half is first attested only at the end of the tenth century, in the Karaite author Yefet ben Eli, and the second appears to have been added later still, reflecting Karaite views already formed.

The consequences of the split are visible across the whole geonic era. Rav Saadia Gaon wrote against the Karaites at length and sharply — and in the course of that polemic created what had not existed before him: a systematic exposition of the principles of faith, a grammar of Hebrew, a translation of the Torah into Arabic. The argument forced explanation, and explanation demanded new genres.