  William introduces his account of the eternity of the world in On the Universe by reference to Aristotle, but he devotes the bulk of his discussion to Avicenna's arguments for this doctrine. In parallel material in On the Trinity he claims that at the root of Avicenna's view lies the principle that “if the one essence is now as it was before when nothing came forth from it, something will not now come forth from it” (Switalski 1976, 69; Teske and Wade 1989, 108). That is, nothing new is caused to exist unless there is a change in the cause. Thus God, a changeless being, cannot start to create the world after not creating it; and therefore he must have created it from eternity and without a beginning. Therefore, the world must itself lack a beginning.