Understanding Aquinas’s Five Proofs for the Existence of God

 In Resources, Saints

The Summa Theologica, written by Saint Thomas Aquinas between the years AD 1265 and 1274, is one of the most extensive summaries of Catholic theology ever written. Meant to serve as a compendium of all known learning of the time, the Summa Theologica still holds valuable insight for understanding Catholic theology. 

One of the many “questions,” or segments, posed in the book is the question of God’s existence. Specifically, Saint Thomas Aquinas examines rational proofs for the existence of God, using five examples to make his point. 

Proof 1: The Argument from Motion 

Saint Thomas’s first proof is based principally on cause and effect. To phrase it like Isaac Newton, “Every action has an equal and opposite reaction.” Although Newton wasn’t even born for close to another four hundred years, Aquinas had a good understanding of the same general principle: nothing acts unless it is first acted upon. 

For Saint Thomas, this provokes a question: What is the first thing to put other things in motion?  

While he framed it using terms like motion, there’s still an understanding in modern science that energy is not created or destroyed but is simply moved and transferred. And when Saint Thomas looked at this chain of cause and effect through motion (or potential and kinetic energy), he recognized that at some point, there necessarily had to be something that was not acted upon. Something had to move first, and that something had to have moved without something else moving it. 

This argument is commonly referred to as the argument of “the unmoved mover,” one that points to a source of motion that was not initially acted upon, a movement that began by itself. This source of movement is the one that set all things in motion, and, as such, had to exist prior to and outside the constraints of that which was set in motion. 

This “unmoved mover,” Saint Thomas says, is God. 

Proof 2: The Argument from Efficient Cause 

Saint Thomas’s second argument is based on the idea of causality. Similar to the first argument, the argument from “efficient cause” is based on the idea of cause and effect. This time, the cause and effect Saint Thomas refers to is existence itself. 

Nothing, he explains, can cause itself to exist. “In the world of sense we find there is an order of efficient causes. There is no case known . . . in which a thing is found to be the efficient cause of itself” (Summa Theologica I, q. 2, a. 3, co.). 

To be the cause of itself, the argument proceeds, the object in question would need to exist prior to its own existence. And because that isn’t possible, everything that exists must have been brought into existence by something else.  

This, of course, cannot go on infinitely. To see causes as infinitely reaching, Saint Thomas says, would be to inevitably arrive at absurdity. The universe is not infinite, and the cause and effect that we see in existence had to have a beginning. 

This beginning, or “efficient cause,” is referred to by humanity as God. 

Proof 3: The Argument from Necessary Being 

The saint’s third argument is an examination of “possibility and necessity.” 

The core of this argument lies in the possibility of nonexistence. “We find in nature things that are possible to be and not to be” (Summa Theologica I, q. 2, a. 3, co.), Saint Thomas tells us. Everything we see has the potential to not exist. This is true of all things in the natural world. 

Following this, Aquinas examines the necessary conclusion reached by this possibility: It is impossible for things with a potential for nonexistence to have always existed. He argues that something that has an end, even if it’s only potential (i.e. the thing still exists), must have had a beginning. 

From here, Saint Thomas draws into infinity: If everything that exists had a point in time where it didn’t exist, then (following his previous argument of cause and effect) nothing would exist now, since there would have been nothing to bring something else into existence. This means that something has to exist that has no potential for nonexistence, and something that is necessary in and of itself. 

This necessary being was not brought into existence, since it has no possibility for nonexistence. It is without beginning, and it is (again, back to Proof 2) the cause of all things that exist. 

This necessary being, Aquinas says, is God. 

Proof 4: The Argument from Gradation 

Saint Thomas observes the natural order as having qualities of goodness, truth, nobility, and so on. The things in the natural world are “more” or “less” good, they are “more” or “less” true, and the same with all similar qualities of nobility, complexity, etc. 

These qualities, he observes, are necessarily compared to the “maximum” of that quality; “a thing is said to be hotter,” he says, “according as it more nearly resembles that which is hottest” (Summa Theologica I, q. 2, a. 3, co.). Every quality has a maximum, to which all things possessing that quality must be compared. 

Something is cold or warm in that it is relatively closer to the lowest possible temperature or the highest. Likewise, Aquinas argues, there must be something that is maximally good, beautiful, and that exists in the highest possible form. 

Again, he observes, this is what we call God. 

Proof 5: The Argument from Design 

The fifth and final proof in this article of the Summa Theologica is taken from the design of natural law, what we refer to today generally as the laws of physics. 

Things without intelligence (which Aquinas calls “natural bodies,” by which he means anything that lacks intellect, such as nonhuman animals, plants, and inanimate objects) “act for an end,” which is a way of saying that they follow the laws of the natural world. These things act in the same way almost always, unless acted upon by a different force, to achieve the same end. 

A rock, dropped from a certain height, will always be pulled by gravity in the same way toward the ground. Likewise, all natural things follow the laws set out for them, not “fortuitously” as Aquinas explains, but by design. 

And if all things without intellect reach their end “by design,” then there is a necessity that there exists an intelligent being that directs these nonintellectual things. 

The forces of the natural world work together in such harmony that, to Saint Thomas, there must be an intelligent force behind them to direct them to their ends. Out of chaos came order, and order requires intelligence. 


These five proofs of Saint Thomas Aquinas have been consistently referenced by Catholics since he wrote them. They succinctly summarize the rational arguments for God’s existence, solidifying the Church’s reliance on both faith and reason as sources for truth. 

Part of Aquinas’s goal with his Summa Theologica was to argue for the teachings of the Church using logical arguments to affirm theological teachings in a way that someone without any faith or experience with religion would be able to grapple with and understand. The arguments of the five proofs are rooted in this approach, which is one of the reasons they still hold so much value to this day. 

“Aquinas’ Five Proofs for the Existence of God” resource from The Catholic Faith Handbook for Youth can be found here.

(The quotations in this post are from the Summa Theologica, by Thomas Aquinas, translated by Fathers of the English Dominican Province [Notre Dame, IN: Christian Classics, 1981].) 
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