The doctrine that man's soul is mortal or, more specifically, that the souls of all men are part of a single soul-substance out of which individuals arise at birth and into which they return at death. The name comes from (Ibn Rushd) Averroes (1126-98), a learned jurist of Cordova who became friend and physician to the ruling caliph when Islam was ascendant. For three centuries Islamic Ash'arite philosophers had denied causality in nature on the ground that it implies the presence of principles other than God in the universe, so making God less than supreme. Averroes saw clearly that the denial of subsidiary causes endangers all knowledge and even reason itself. In support of this view he appealed to Aristotle, who had accepted causality wholeheartedly.

Averroes wrote commentaries on Aristotle* and came to be known to posterity as “The Commentator.” To reconcile his views with Islam he interpreted the Koran allegorically and perhaps not overseriously; in late life he was accused of heresy. Apart from his views on immortality and the soul, Averroes identified God with Aristotle's remote and impersonal Prime Mover, denied free will, and taught that both the world and mankind are eternal. In 1253 Aristotle was prescribed for study by the University of Paris, and the Commentaries of Averroes were accepted as the standard text. There was consternation in Christendom when many students accepted their anti-Christian teachings, and the study of Aristotle was prohibited by Urban IV in 1263. Albert the Great in 1256 and Thomas Aquinas in 1257 and again in 1270 wrote works directed against the Averroist heresies, but despite prohibitions the latter survived in places, notably in Padua to the time of the Renaissance.

See M. Fakhry, Islamic Occasionalism (1958).