Thomas Aquinas’s Five Proofs for God Revisited


Trent Horn

Article ID:



Mar 10, 2023


Aug 11, 2020

This article first appeared in the Christian Research Journal, volume 41, number 2 (2018). For further information or to subscribe to the Christian Research Journal please click here.

In the thirteenth century, Thomas Aquinas revitalized Christian theology by applying principles of Greek philosophy to the explanation and defense of the Christian faith. Thomism, or the philosophical application of Aquinas’s thought, has a privileged place in the Catholic Church and has been embraced by a growing number of “Evangelical Thomists.”1 Among non-Christians, Aquinas is usually encountered in first-year philosophy textbooks via excerpts of his five ways of proving the existence of God from his Summa Theologica.

Since the Summa was intended to be an introductory guide to theology, the five ways constitute only a few pages of text, which is perhaps why Richard Dawkins writes of them in The God Delusion: “[They] don’t prove anything, and are easily — though I hesitate to say so, given his eminence — exposed as vacuous.”2 I intend to argue that Dawkins and critics like him think the five ways are vacuous proofs because they misunderstand them. While space does not permit a full-fledged analysis of these arguments, we can correct common misperceptions many critics have about them.


Dawkins summarizes Aquinas’s first way of proving the existence of God as follows, “Nothing moves without a prior mover. This leads us to a regress, from which the only escape is God. Something had to make the first move, and that something we call God.”3

But Aquinas never makes the blanket statement, “nothing moves without a prior mover.” That would be on par with saying, “Everything has a cause,” to which atheists will object, “But if everything has a cause, then what caused God!?” Rather, Aquinas begins with a principle from Aristotle that explains how change and motion are possible.

Some ancient Greek philosophers, such as Parmenides, claimed change was an illusion because something either exists or it doesn’t exist. If a bowl of water is liquid, then no ice exists in the bowl. However, if the water changes into ice, that would mean the ice (which previously did not exist) would now exist. Such a scenario would then involve something coming from nothing, which is impossible. Therefore, change is an illusion.

Aristotle’s answer to Parmenides is that the ice does exist in the bowl of water as a potential not actual thing. The ice does not come from nothing but exists as potential in the water. It is then brought into actual existence by something else, such as air pressure.4

In the argument from motion, Thomas says that it is impossible for something to exist as both a potential and an actual entity in the same respect. For example, no object can move itself, because the actualization of movement would be caused by the potential for movement, both of which can’t exist at the same time. This means the potential for change or motion that is actualized in the object must have been caused by something else.

Aquinas then says, “This cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover….Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”5

This chain of actualizing potential cannot be infinite because it wouldn’t explain why there was any motion or change at all. This kind of explanation would be on par with explaining the motion of a freight train by saying there were an infinite number of boxcars each pulling the car behind it. Such a train, even if it were infinitely long, would still remain motionless because boxcars only receive motion — they can’t generate it.

The only adequate explanation, then, is the existence of a car that gives motion without receiving it, or in this case, a locomotive. Aquinas concludes that there must be a cause of the universe that is pure actuality and has no potential whatsoever, which he knows as God.


Dawkins says that Aquinas’s second way of proving the existence of God proceeds as follows: “Nothing is caused by itself. Every effect has a prior cause, and again we are pushed back into regress. This has to be terminated by a first cause, which we call God.”6

It seems clear in The God Delusion that Dawkins thinks Aquinas is arguing that the chain of cause and effect cannot extend backward for eternity – what is known as “infinite regress”—and therefore there must have been a beginning of time that has God as its cause. Dawkins then posits that “something” like a “Big Bang singularity” is a simpler, more likely explanation of this first cause.7

However, Aquinas did not believe that one could prove from reason alone that the world had a beginning in time. He is not arguing for a temporal series of causes that terminate in an absolute beginning but in a hierarchical series of causes that terminate in an absolute, uncaused cause. In order to understand Aquinas’s second way, we have to understand the difference between an essentially ordered series and an accidentally ordered series.

An accidentally ordered series is one whose present members are not causally dependent on the actions of past members in the series. Think of a long series of dominoes that are knocked over one by one. This is an accidentally ordered series because the dominoes that already fell do not directly affect the falling of the last domino. They are the reason that domino is falling, but they are not the present cause of its falling. When the current domino in the series topples, all the previous ones that have already fallen could be scooped up and thrown in the trash — they aren’t necessary for the rest of the dominoes in the series to fall.

But now think of a series of gears. Each gear spins because it interlocks with the teeth of the gear behind it that is also spinning. But that gear spins only because it interlocks with a spinning gear located behind it. Unlike the dominoes, this is an essentially ordered series because the previous gears that are spinning directly do affect the motion of the last gear in the series. If any gear in the series were removed, then the entire series would stop moving. Every member is essential for the chain of motion to continue.

Even if the universe were eternal, Aquinas concludes that the continuing existence of objects requires continuing support from other objects (e.g., objects exist because of atoms, atoms exist because of atomic forces, atomic forces exist because of fundamental universal constants, etc.). Like a giant gear in the center of an assembly of smaller gears, Aquinas concludes that there must be a reality that causes and sustains all things without receiving causation or existence from anything else. This “uncaused cause” simply is what many people call God.


Dawkins says that in this third argument proving the existence of God, Aquinas claims, There must have been a time when no physical things existed. But, since physical things exist now, there must have been something non-physical to bring them into existence, and that something we call God.”8 As can be expected in his philosophical analyses, Dawkins has stated the exact opposite of what Aquinas is arguing.

Aquinas notes that things in the world are possible in the sense that they could not exist. If the universe were eternal, then all possible states would be actualized, including one where all things in the world actualize the possibility of not existing. However, if that happened, then nothing would exist in the present because something cannot come from nothing. Therefore, there must be something that is necessary to keep these possible or, to use a modern term, contingent (occurring or existing only if certain other circumstances are the case) beings in existence.

But, according to Aquinas, only one kind of being has necessity or existence as a part of its nature, and that is God.


Aquinas’s fourth way of proving the existence of God is the one that sounds the most foreign to modern ears. He begins by noting that certain things are “more” or “less” good in respect to a maximum example of that goodness. He then writes, “The maximum in any genus is the cause of all in that genus…. 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.”9

Dawkins guffaws at the idea that this is a serious argument and casually dismisses it by saying, “You might as well say, people vary in smelliness but we can make the comparison only by reference to a perfect maximum of conceivable smelliness. Therefore there must exist a preeminently peerless stinker, and we call him God.”10

Aquinas is not saying, as Dawkins alleges, that there exists in reality a maximum to everything we see, and this maximal reality is God. Instead, he is saying that goodness is an objective concept, and what makes a being more or less good is the possession of an objective act of existence, or the possession of being. Malodorous scents and other bad things are examples of a lack of goodness caused by a lack of being. In contrast, when trees grow and human beings mature intellectually, these represent examples of moving toward perfection, or a fullness of being.

This argument also shows that entire classes of beings are better than other classes because they have more existence and more perfection. According to Peter Kreeft and Ronald Tacelli in their defense of this argument, “If these degrees of perfection pertain to being and being is caused in finite creatures, then there must exist a ‘best,’ a source and real standard of all perfections that we belong to us as beings. This absolutely perfect being — the ‘Being of all beings,’ ‘the Perfection of all perfections’ — is God.”11


Dawkins completely misunderstands the fifth argument proving the existence of God because he conflates it with the one made in William Paley’s nineteenth century work Natural Theology. In that work, Paley argued that if you found a watch lying in the sand on a beach, you would conclude (because of the organized complexity of the watch) that rather than originating from the sand itself, it was dropped there by its designer. As such, living creatures are even more complex and “accommodated to their ends” than watches, so God must have designed them. Dawkins simply says that “evolution by natural selection produces an excellent simulacrum of design,” so God isn’t necessary.12

But Aquinas’s fifth way relies not on the organized complexity of an artifact as proof of design but order and regularity simpliciter. He writes, “Things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result.”13 Aquinas then says that just as an arrow cannot hit a target without an archer, unintelligent forces cannot achieve their regular ends without an intelligent cause, which is God.


We should remember that not all of these arguments will be persuasive equally to people. Aquinas even said that the first way, or the argument from motion, is “the most manifest” way of proving the existence of God. Space also does not allow us to examine every objection to these arguments or the various replies given by modern philosophers who defend them.14 But one last objection from Dawkins is worth mentioning. He writes, “There is absolutely no need to endow that [final cause in Aquinas’s arguments] with any of the properties normally ascribed to God.”15

In the Summa Theologica, Aquinas devotes considerable space to explicating the attributes of the ultimate cause of the universe. He reasons that if this cause is pure actuality and has no potential, then it cannot be made of matter or exist in time lest it have spatial or temporal potentiality. Along with being immaterial and eternal, this cause must be infinite because finitude implies a potential that has not been reached. This also means the cause is omnipotent, since an inability to do something would represent unactualized potential.  Moreover, because it is the cause of all perfections and order in the universe and because evil is an absence of good, this cause must be omniscient, personal, and omnibenevolent, since it has no deficiency in its existence.

Aquinas says of this infinite, immaterial, eternal, all-powerful, all-knowing, and all-good reality, “This all men speak of as God.”16

Trent Horn is on staff with Catholic Answers and the author of five books, including Answering Atheism (Catholic Answers Press, 2013) and Persuasive Pro-life (Catholic Answers Press, 2014).


  1. See, for example, Norman L. Geisler, Thomas Aquinas: An Evangelical Appraisal (Grand Rapids: Baker, 1991).
  2. Richard Dawkins, The God Delusion (New York: Houghton Mifflin Company, 2006), 77.
  3. Dawkins, The God Delusion, 77.
  4. See Book One of Aristotle’s Physics.
  5. Summa Theologica, First Part, Question 2, Article 3.
  6. Dawkins, The God Delusion, 77.
  7. Dawkins, The God Delusion, 78.
  8. Dawkins, The God Delusion, 77.
  9. Summa Theologica, First Part, Question 2, Article 3.
  10. Dawkins, The God Delusion, 78.
  11. Peter Kreeft and Ronald Tacelli, Handbook of Christian Apologetics (Downers Grove, IL: InterVarsity Press, 1994), 55.
  12. Dawkins, The God Delusion, 79.
  13. Summa Theologica, First Part, Question 2, Article 3.
  14. For a more in-depth treatment, see Edward Feser, Aquinas: A Beginner’s Guide (Oneworld, 2009).
  15. Dawkins, The God Delusion, 77.
  16. Summa Theologica, First Part, Question 2, Article 3.
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