Why the Five Ways?: Aquinas’s Avicennian Insight into the Problem of Unity in the Aristotelian Metaphysics and Sacra Doctrina

This paper will argue that the order and the unity of St. Thomas Aquinas’s five ways can be elucidated through a consideration of St. Thomas’s appropriation of an Avicennian insight that he used to order and unify the wisdom of the Aristotelian and Abrahamic philosophical traditions towards the existence of God. I will begin with a central aporia from Aristotle’s Metaphysics. Aristotle says that the science of first philosophy has three different theoretical vectors: ontology, aitiology, and theology. But how can all three be united into a single Aristotelian science? In his Metaphysics of the Healing, Avicenna resolved the impasse by taking the ontological vector as the subject of metaphysics. He then integrated the question of the four first causes into the penultimate stage of his demonstration for the existence of God, thereby placing aitiological and theological questions among the ultimate concerns of a unified Aristotelian metaphysics. In the five ways, St. Thomas integrated Avicenna’s Aristotelian search for the first four causes into the last four of his five ways, by showing that each of the four aitiological orders terminate in an ultimate first cause that we call God. Finally, by appending the proof from the Physics to the beginning of the five ways, St. Thomas was able to show that the ultimate aim of both natural philosophy and metaphysics is the divine first principle, which is the beginning and subject of sacra doctrina