Thomas Aquinas and Proof of God

By Pierce K. Kozlowski

St. Thomas Aquinas (1225-1274)—an Italian Dominican friar, philosopher, Catholic priest, and Doctor of the Church—understood that God had existed, but struggled to explain God’s existence through the laws of the natural world. To bridge the gap between God and the natural world, Aquinas developed five cases that attempted to prove God’s existence using undeniable facts of the universe in his book, Summa Theologica.

Five Cases for God

Aquinas’ first case is the argument from the Unmoved Mover. This case revolves around the idea that physical actions prompt other physical actions to occur. If someone were to trace every cause for every action that has ever happened, there would be a stopping point. Aquinas wrote, “this [process] cannot go on to infinity, because then there would be no first mover . . . subsequent movers move only inasmuch that they are moved by the first mover. . . Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.” Aquinas noted that something must have started movement, and that something—or someone—is God. This same principle is applied in Aquinas’ second case as well, which is the argument from Causation, and it argues that God was the first cause to begin the chain of cause and effect.

The third case made by Aquinas is the Argument from Contingency. This case argues that all things depend on something for their existence. A tree cannot grow without water and sunlight, and a person cannot be born without their parents. Aquinas logically concluded that creation must therefore rest on something uncreated, upon which everything rests, and that was God. Aquinas writes, “That which does not exist begins to exist only through something already existing . . . Therefore we . . . admit the existence of some being having . . . its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.” If this is not true, how can creation’s existence rest on nothing if everything in nature rests on something for its existence? It cannot. Just as the birth of a child is contingent on the mother, creation’s existence is contingent on God.

Aquinas’ fourth case is the argument from degree. This case argues the quality of things being good or bad, and better and worse. To have a metric of this kind, there must therefore be a state of the highest good to measure by. Because there is such a degree of perfection and righteousness, it can therefore be obtained by a being that exists, and that being is God. Aquinas writes, “fire, which is the maximum of heat, is the cause of all hot things . . . Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.” Aquinas noted that just as hot things in the universe must come from heat, good and righteous things in creation must come from God.

Aquinas’ fifth and final case is the argument from the final cause. This case argues from the teleological standpoint, which is that things in nature operate in predictable ways and work towards predictable ends. A tree sapling works to grow into a tree according to the species of the sapling, even if the sapling is met with failure. Aquinas concluded that there must be a force that directs beings towards their final destination, and because such a force would have to be a wise and intelligent being, that force would be God. Just as the arrow is directed by the archer, natural beings—or people—are directed by God.

The efforts of Aquinas to prove the existence of God can be boiled down to this: Because God is the master of the Heavens and all creation (Psalm 115), our existence within creation is, therefore, proof of God.

References

1. McInerny, Ralph, and John O’Callaghan. “Saint Thomas Aquinas.” Stanford Encyclopedia of Philosophy, Stanford University, 23 May 2014, plato.stanford.edu/entries/aquinas/.

2. Aquinas, St. Thomas. Summa theologica Vol. 1-V. 1485

3. "Psalm 115". CSB Bible- - Bible Gateway, www.biblegateway.com/passage/?search=psalm%2B115&version=CSB.