SA'ADJĀH ben JŌSĒPH. — Gaon, born at Fajjūm in 882, died 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 thirty 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 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 Islamic theories opposed to biblical Revelation. A follower of the kalām in its rationalistic Mu'tazilite aspect, he promotes the concept of God's unity and of free will in man. Religious truth is that revealed to Israel; reason is the common basis of the most diverse religions, which are due to men rather than to God. Revelation pursues an educative purpose without wishing to annul man's 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. That is true which proves historically true to all.
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 continual 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 hereafter as an element of recompense. 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 messianic redemption.
In addition to the works mentioned above, the following should be recalled: 'Aghrōn, a kind of Hebrew dictionary for the use of rhymers, with an appended study of the various forms of poetry; The Books on Language, the oldest grammar to have come down to us; a commentary on the 70 hapax legōmena of the Bible; and, against the Karaites: The Book of the Refutation of 'Anon and The Book of Distinction.
Many of S.'s works were translated into Hebrew and into various European languages.