  A few other sources are named in Yesod Mora: Shi‘ur Qoma, a cryptic writing that, like Sefer Yesira, nourished a good deal of speculation, much of it mystical, and a certain Sefer ha-Yihud, which is not extant. These works date to the gaonic period which, in Jewish historiography, corresponds to very late antiquity and the early Middle Ages. All in all, then, the sources that Ibn Ezra cites by name are a mix of gaonic kalam and proto-kabbalistic mysticism. Ibn Ezra adopted the literary format of early gaonic texts for those of his writings that can be called philosophical.  Yesod Mora and Sefer ha-Shem, “The Book of the [Divine] Name”, are dense and cryptic, much like Sefer Yesira and Shi‘ur Qoma.