 The Greek theory of the eternity of the world was a challenge to the Christian world view, a challenge increased by the fact that the theory was supported by Arabic sources: by Avicenna, who combined it with a metaphysical concept of God as the “necessary being in itself”, which is the eternal efficient cause of the existence of the heavenly and sublunar world (see section 6.4 on the first cause), and by Averroes, who combined it with a conception of God as the prime mover, whose existence is proved in natural philosophy. In Giles of Rome’ Errors of the Philosophers, Avicenna and Averroes are accused of many “errors”, but the eternity of the world figures most prominently. Averroes, in particular, is attacked for opposing “even more vehemently than did the Philosopher (Aristotle) those who held that the world had had a beginning” (Giles of Rome 1944, 15). The eternity of the world and related theses were condemned as heretical in 1270 and 1277 by the Parisian bishop Étienne Tempier (art. 87, 90, 99, 184) (Piché 1999).