Ancient Greek philosophers have much to say about God or the gods; some of them also have much to say about being (whether being as predication or identity, expressed by “X is Y,” or being as existence, expressed by a bare “X is” or “there is an X”). They do not systematically connect the two topics, however, and neither do many modern philosophers. But many medieval philosophers did. Can thinking about being help us understand God? Can thinking about God help us understand being? I will explore some connections that medieval philosophers saw between the two topics, and also some difficulties that they encountered. I will focus not so much on particular philosophers as on central ideas that many different philosophers took up, illustrating these ideas from the work of philosophers who set them out in especially interesting or accessible ways, and noting challenges that different philosophers answered in different ways. Many of these ideas and challenges begin with Muslim authors and are then taken up by Christian authors from the thirteenth century on. I will go back and forth between Muslim and Christian sources.

PHYSICAL AND METAPHYSICAL PROOFS OF GOD

The proofs of the existence of God are an obvious place to begin. Thomas Aquinas, in Summa theologiae I, q. 2, a. 3, says that God’s existence can be proved in five ways. Thomas’s first way, arguing from causes of motion, and his second, from efficient causes, are physical arguments, taken from Aristotle; his fifth way, from teleology, is equally physical, derived ultimately from the Stoics.