SA'ADJAH BEN JŌSĒPH. - Gaonite, b. at Fajjūm in 882, d. in 942. He was a grammarian, exegete, translator of the Bible into Arabic, theologian, polemicist, and founder of medieval Jewish religious philosophy. Until the age of 30 he lived in Egypt, then in Palestine, Syria and Babylonia, where in 928 he was appointed head of the Talmudic Academy of Sūrā.
In 931 he completed his commentary on the Sēpher jēšrāh, and in 933 his principal philosophical work: Book of the Opinions and Religious Doctrines (in Arabic: Kitāb al-amānāt wa 'l-i'tigādāt; in Hebrew: Sēpher ha-'emūnōth wē-ha-dē'ōth). His biblical commentaries too are philosophical in tone. Everywhere he opposes the Christian Trinitarian concept, Zoroastrian dualism, and the Islamic theories opposed to biblical Revelation. A follower of the kalām in its Mu'tazilite rationalistic aspect, he favors the concept of the unity of God and of free will in man. Religious truth is that revealed to Israel; reason is the common basis of the most varied religions, which are due to men rather than to God. Revelation pursues an educational purpose without wishing to annul in man his rational activity, which aims to demonstrate the truth of what was revealed by God himself and preserved by historical tradition, which derives from the collective awareness that excludes every illusion and every error. Whatever proves to all to be historically true is true.
The existence of the world presupposes the existence of the Creator, and creation is a form of Revelation. The world is composed of parts subject to continuous becoming and perishing. Accidents arise in the flow of time and are limited by time itself. The world is not eternal. Morality, willed by God, aims to make man happy (eudemonism). Theodicy presupposes the afterlife as an element of equalization. Suffering is a means of purification and a preparation for humility and well-being at a later time. The resurrection presupposes the reunion of the soul with the body and will coincide with the time of the Messianic redemption.
In addition to the aforementioned, mention should be made of: 'Aghrōn, a kind of Hebrew dictionary for the use of versifiers, with an appended study on the various forms of poetry; The Books on Language, the oldest grammar that has come down to us; a commentary on the 70 hapax legōmena of the Bible; against the Karaites: The Book of the Confutation of 'Anon and The Book of Distinction.
Many of S.'s works were translated into Hebrew and into various European languages.