The Medieval theologian/philosophers endeavored to “demonstrate through logic and argument what [they] believe by faith” (Cleveland 163). Two key philosopher/theologians, St. Anselm of Canterbury and St. Thomas Aquinas, utilized philosophical categories to logically defend their faith in the God revealed in Scripture to further their own understanding of the God they confessed. St. Anselm of Canterbury proposed the famous “ontological argument” while St. Thomas Aquinas developed his “five ways”.

St. Anselm presents his ontological argument in the form of a prayer in his work, Proslogion. He begins his prayer with a yearning to know God, yet recognizes he is far from God in his fallen state. He pleads for God to help him in his journey to seek after God and know Him (Allen and Springsted 84-86). The ontological argument is then presented as a tool for him to move from belief in God to an understanding of God (86).

The ontological argument begins with a definition of God: “a being than which nothing greater can be conceived” (86). Even the “fool” who denies God (Psalm 14:1) can understand the definition in his or her mind (Allen and Springsted 87). Yet if such a being is that “than which nothing greater can be conceived”, it must exist in reality since it is greater to exist in reality than merely in the understanding. A being that only exists in the mind, but not in reality would not be that “than which nothing greater can be conceived” (87). Through the ontological argument, St. Anselm argues from who God is revealed to be in order to prove why God must also exist. He begins with the God he confesses and utilizes his understanding to show why such a God must exist.

St. Thomas Aquinas, on the other hand, rather than argue from who God is to God’s existence, argues from the actions of God seen in creation to his existence. St. Thomas Aquinas argues that because God’s existence is accepted on faith, it can also be rejected. Because God’s existence can be rejected, God’s existence is not self-evident. The ontological argument does not work if God’s existence is not self-evident and is rejected by St. Thomas Aquinas (Summa Theologica I. Q2. A1). St. Thomas Aquinas does not believe God’s essence can be known and, so, argues from God’s effects (I. Q2. A1), the opposite direction of St. Anselm of Canterbury’s argument. Because all effects have a cause, humans can reason back from the effect to the cause and see that God does, in fact, exist (I. Q2. A2.).

St. Thomas Aquinas’s five proofs are not designed to convince unbelievers of God’s existence. He explicitly states, “If our opponent believes nothing of divine revelation, there is no longer any means of proving the articles of faith by reasoning, but only of answering his objections-if he has any-against faith” (I. Q1. A8.). The five proofs begin in revelation and use reason to explain the existence of God based on God’s revelation (I. Q1. A8.), each of which will be considered in turn.

First, St. Thomas Aquinas observed that there are objects in the world in motion. For something to be in motion, it must be “put in motion by another”. Using Aristotelean language, all these objects have the potential for motion in themselves, but a cause is necessary to move them in actuality. A thing cannot move on its own. Since there are not infinite causes of motion, there must be a first mover that does not require another to put it in motion from which all motion originates. St. Thomas Aquinas identifies the first mover as God (I. Q2. A3.).

St. Thomas Aquinas’s second way he calls the “efficient cause”. Similar to the first way, he argues that there are not an infinite number of causes. There must be a first cause that was itself uncaused. He, again, identifies the first, efficient cause as God (I. Q2. A3.).

In the third way, he argues that there are things “that are possible to be and not to be”. Things that could not exist have not always existed. Since all things are possible not to be, at some time there was nothing. Since nothing can cause existence, there must be something whose existence is necessary. It depends on nothing else to exist, but all things depend on it for their own existence. This necessity must be God (I. Q2. A3.).

Fourth, there are things that are “more and…less good, true, noble, and the like”. Yet for there to be gradations of goodness, there must be a most good, true, and noble to which all other things are compared. God, then, is that which is most good, true, and noble (I. Q2. A3.).

Finally, there are unintelligent objects in the universe that act in a consistent way, such as the revolutions and rotations of the planets. Since these objects are not intelligent, something external to them must act on them for them to move towards a consistent end. God must act on them to direct them towards an ultimate end (I. Q2. A3.).

Both St. Anselm of Canterbury and St. Thomas Aquinas have the same goal: to use reason to explain the existence of God, a truth they both already affirm by faith in God’s revelation. However, they do so in different ways, arguing in the opposite direction. St. Anselm argues from the top-down, from who God is, to prove His existence. St. Thomas Aquinas argues from the bottom up, from the effects of God’s activity in the world, to prove that His existence. Both approaches can reinforce for Christians a belief in the existence of God, adding certainty to the faith they already confess.

Bibliography

Allen, Diogenes, and Eric O. Springsted, eds. Primary Readings in Philosophy for Understanding Theology. Milton Keynes UK: Lightning Source UK, 2010.

Cleveland, Christopher. “The Goal of Philosophy in Medieval Theology”, Philosophy and the Christian: The Quest for Wisdom in the Light of Christ. Edited by Joseph Minich. Lincoln, NE: Davenant Press, 2018.

St. Thomas Aquinas, Summa Theologica.