Thomas Aquinas’ 5 Arguments for God: Natural Reasoning


Have you ever wondered why the sky is blue? You know something is true - namely, the sky is blue - yet might not be able to explain it. Philosophers ask questions to understand how we know what we know. Science, or observation of the world, is one way philosophers examine knowledge. Thomas Aquinas was a 13th-century Christian philosopher who had faith in God. However, he struggled to explain why God is real. 


Thomas Aquinas sought proof of God's existence by examining the natural world. He wanted to use natural laws to explain why God was real. Each of his five arguments, called the cosmological arguments, originates from a concept in the cosmos that needs an explanation. Aquinas provided five cases to demonstrate God's existence through undeniable facts of the universe in his book Summa Theologica

The Argument of the Unmoved Mover: Aquinas's first argument relates to motion. He observed that in the world, physical actions are the only thing that causes other movements. He used the example of fire and wood. Fire changes the energy in wood, which moves it to become hot. Wood cannot become hot without fire. Aquinas applies this law to the world. Something must have started movement in the first place, and not have been caused to move by something else. To Aquinas, that something is God. 

The Argument of the First Cause: The second argument, which is similar to the first, is the argument of causation. Aquinas found that everything in the natural world has a cause. For example, in a domino chain, each domino that falls causes another domino to fall. Aquinas believes God must have started the cause and effect chain. 

The Argument from Contingency: In this third argument, Aquinas finds that things in the natural world depend on other things for their existence. You could not be born without your parents. A tree cannot grow without receiving sunlight and water. Aquinas thought there must be something not dependent on other things for its existence, and upon which everything else rests on for its own existence; for Thomas Aquinas, that thing is God. 

The Argument from Degree: The fourth argument is about degrees of good things. Aquinas argued that we need a scale to measure the value of things as bigger or smaller, greater or lesser, and better as opposed to worse. To have a measurement system, we need something of the highest perfection, goodness, and truth to measure by and from which to obtain those qualities, which is God. 

The Argument from Final Cause: The last argument is of completion or the teleological argument. Aquinas observed that everything in nature moved according to predictable ways and towards predictable ends. For example, acorns always move towards becoming oak trees, even if they don't succeed. Aquinas thought God must be directing beings to their final goal since something intelligent needed to do so.

Influences: Past and Future

Other philosophers such as Anselm and Aristotle influenced Aquinas's arguments. He wrote these five defenses as a response to Anselm's Ontological Argument, a singular proof of God, which Aquinas did not find convincing. Aristotle's philosophy of looking for truth in the material world, view of time and motion, and cosmology inspired the methods and concepts that Aquinas used to prove God's existence. 

In a critique of Aquinas's arguments, philosophers such as David Hume and Emanuel Kant found the first three self-defeating. For example, in the first argument, the concept of an Unmoved Mover undermines the argument's premise. Philosophers are prompted to question why God is the exception to a fundamental law of the universe, and if God made everything, who made God. Hume inquired why it's impossible for the universe to be an infinite series of causes and effects in motion. He used examples like finding the largest prime number to illustrate how infinite series exist. Kant further explained the flaw of the second argument by stating there is no way to know that the final causes are reached. 

Why Care?

Thomas Aquinas's legacy rests in his Sainthood and position as a celebrated doctor of the church for his work during a time of radical discovery. Aquinas observed and questioned the natural world, which we can also do in asking how others assume the world works. 

Aquinas shows that we can accept a conclusion without agreeing with its reasoning, or agree in part with the premise or reasoning but reject the conclusion. For example, followers of various religions agree that God exists but have opposing ideas on how that impacts their life choices. Like Thomas Aquinas has five separate arguments, we can know something is true for different reasons.

Think Further

  1. Which of the five arguments is most convincing to you? Least convincing? Why?
  2. What might have motivated Thomas Aquinas to write five proofs instead of one? 
  3. If Thomas Aquinas already believed in God, why would he need to prove God’s existence?


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Learn More

  1. Crashcourse. “Aquinas & the Cosmological Arguments: Crash Course Philosophy #10.” Youtube, 11 April 2016.
  2. McInerny, Ralph, and John O’Callaghan. “Saint Thomas Aquinas.” Stanford Encyclopedia of Philosophy, Stanford University, 23 May 2014,
  3. Thomas, Aquinas, Saint, 1225?-1274. The “Summa Theologica” of St. Thomas Aquinas. London. Burns, Oates & Washburne, ltd., 192042.
  4. Stefon, Matt. “The Five Ways.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 15 May 2020,
  5. Wallis, Charles. “Thomas Aquinas and the Five Ways.” California State University, Long Beach.
  6. Wood, William. “Thomas Aquinas on the Claim that God is Truth.” Journal of the History of Philosophy, vol. 51 no. 1, 2013, p. 21-47. Project MUSE, doi:10.1353/hph.2013.0007.