 Thomas Aquinas adopts from Avicenna the description of God as the necessary being, whose essence is its own existence (Summa contra gentiles I.22). Avicenna’s influence is also apparent in a well-known passage: the “five ways” (quinque viae) of proving God’s existence in the Summa theologiae (Ia q. 2 a. 3). The third proof advances via “the possible and the necessary”: It establishes the existence of something necessary in virtue of itself. The proof is much coloured by Avicennian metaphysics, but its direct source is Maimonides’ Guide for the Perplexed (II.1), from which it deviates only in minor respects. A conspicuous element of Maimonides’ and Thomas’ proof is the premise, not explicitly formulated by Avicenna, that, in an eternal world, every possibility is eventually realized, which is a version of the so-called “principle of plenitude” (Davidson 1987, 378–388). Thomas, however, rejects the Avicennian theory of emanation. The creation of the world is not a necessary process, and it does not occur through intermediaries, that is, intelligences or angels. He also rejects Avicenna’s theory of a separate active intellect as the giver of the forms of the sublunar world and as the ontological place of the intelligibles, a theory against which Thomas advances epistemological arguments in the first place (Summa contra gentiles II.42 and II.76). Albertus Magnus, in contrast, tacitly adopts much of the dator formarum theory in several of his writings, even though he often voices his criticism of it (Hasse 2012).