Introduction

Medieval philosophical discussions of the existence of God would not, in general, concern the question whether God exists as such so much as whether God’s existence can be established indubitably by an argument, and if so, how. The problems discussed were therefore intimately connected to the question of the relative significance of faith and understanding in the life of the religious person. In dealing with the subject, thinkers from the three great monotheistic religions could find ample sources of inspiration outside their own cultural spheres. Doctrines such as the triunity of God, for instance, particular to Christianity, was by most maintained to be a matter of faith, and was as a rule treated as a separate issue. By contrast, we find in all three traditions arguments to the existence of God based on the notion that God is the first cause of our universe of causes and effects.

Early attempts at proving God’s existence drew inspiration primarily from Platonist philosophy. Later Aristotle came to dominate as an influence, both indirectly as a target for attacks from Christian and Islamic theologians and as a direct source among Arabic philosophers and Christian scholastics. Aristotelian philosophy enriched the discussion about how to deal philosophically with the question of God’s existence in at least four important ways: (1) by confronting philosophers with a certain metaphysical framework against the background of which the concept of existence could be assessed, (2) by supplying a theory of human cognition, (3) by providing methodological discussions about the validity of arguments and what is required of scientific demonstration, and, finally, (4) by supplying, in the Physics and in the Metaphysics, actual examples of proofs for God’s existence in the form of arguments for a first mover.

The arguments for the existence of God may be seen as loosely arranging themselves around three basic ideas: first, that of the world consisting in a hierarchy of beings with respect to intrinsic value, secondly, that of God as a primary creative cause, and, thirdly, that of the universe being manifestly orderly and purposeful. These patterns of thought gave rise to arguments that are in the philosophical literature after Kant considered as falling into three classes of proof: the ontological, the cosmological, and the teleological. While this classification may serve as a rough guide, one should note that some medieval arguments draw on more than one of the three basic ideas.