Do the Five Ways of Saint Thomas Prove the Existence of the Christian God?
The most famous and widely cited and critiqued proofs for the existence of God are the Five Ways of Saint Thomas Aquinas. Yet, atheists, and even other theistic critics of Aquinas, are quick to point out that whatever cause whose existence is supposed to be proved, is far less than the God of the Bible (Jesus or Yahweh).
Even if they are successful, critics allege, all they prove is that five features of the world have an ultimate explanation, but not what the explanation is, or even that the same thing serves as the ultimate explanation for each diverse feature, much less what it is like (if there is this one thing), or that it is the same as what any religion worships, least of all the God of Christians. They note, however, that Aquinas ends each of the Five Ways with a statement to the effect of “This everyone calls God.” Critics thus fault Aquinas for making the unwarranted claim that his Five Ways prove that God (his Christian God) exists and not that just some unknown First Cause (or set of causes) exists.
In making these critiques it is as though these atheists (and others) have failed to read even the few pages that come before and after the text of the Five Ways, much less anything else Aquinas has written. But, thankfully, you have this essay to help you understand Aquinas better.
Proof of the Christian God?
First, how can Aquinas claim his Five Ways prove that God (his Christian God) exists and not that just some unknown First Cause(s) exists?
The Five Ways are from Part I of Aquinas’ Summa Theologiae, Question 2, Article 3. But in the very First Question, in the very first Article, Aquinas tells us that what Christians believe about God as unique to their religious faith cannot be proved through philosophical argument, but only comes through revelation, i.e. the Bible. “Hence it was necessary for the salvation of man that certain truths which exceed human reason (emphasis added) should be made known to him by divine revelation.” And in Article 6, Reply to Objection 2, he says, “But the knowledge proper to this science [concerning Who God is in Himself] comes through revelation and not through natural reason.” For Aquinas, we can know by unaided reason some things about God: that He exists and something of His nature: that He is not material, that He is good and perfect, that He is all-powerful and all-knowing. But some truths about God can only be known because He has revealed Himself in the Bible, and principally in the person of Jesus, the Eternal Son of God, Second Person of the Holy Trinity.
Moreover, Aquinas explicitly says, in the very article just prior to the Five Ways, in Question 2 Article 2, Reply to Objection 1 , “The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles.”
It is unfair then to see it as a failure that Aquinas’ philosophical proof does not establish the Christian message that God became man in Jesus, when Aquinas has just told his readers that he does not believe that it is even possible to established such Christian truths by a philosophical proof.
So, Aquinas is under no illusion that the Five Ways prove that the cause of the universe is the Christian God, though Aquinas, obviously, does believe that the cause of the universe is the Christian God. Aquinas only claims to show that there is a First Necessary Uncaused (Perfect and Ordering) Cause of the universe. Aquinas believes he knows the identity of this Cause (the Christian God as revealed in the Bible) even if he does not think he has, or even that he can, philosophically prove Its identity.
Which God is proved?
How does that work? How can one know or prove something exists without knowing what it is? This brings us to the second critique: Which “god” do the proofs establish? If the Five Ways prove some cause exists, why is it a god and which god is it?
This sounds a lot more troubling than it is. Actually, in science, this happens all the time, since as Aquinas also tells us in the very article prior to the Five Ways, Question 2 Article 2, Reply to Objection 2, that one has to know whether a thing is (its existence) before one can know what it is (its essence).
In terms of language, philosophers now-a-days distinguish between words or names having a referent and their having a meaning or definite description. The referent is the thing the word “points to” and the description is what about the thing is understood, and the basis for the pointing. And one and the same thing can have many and various descriptions – yet all the descriptions refer to the same thing (referent), though the descriptions are different.
Examples are common: the description “Xanthippe’s husband” and the name “Socrates” both have the same referent or both “point to” the same existing individual: an annoying Athenian philosopher who was executed in 499 BC.
Or, another example, Phosphorus “the Evening Star” and Hesperus “the Morning Star” both refer to the planet Venus even though each describe it in terms of different times of day and in different parts of the sky.
And some descriptions are performative – the descriptions do not list attributes of the thing or person so much as that thing’s relation to other things. As “the winner of the race” identifies the runner who crosses the finish line first, without providing any further description of who that runner might be. The effect that a thing has on others is another instructive example of a performative description. One can refer to a thing by discovering that it causes an effect on something else.
Take the example of “Roxanne’s Beloved” – In the story of Cyrano de Bergerac (which, if you don’t know, you should get to know), a hideously long-nosed poet (Cyrano) secretly longs and pines for an impossibly beautiful woman (Roxanne). But since she is seemingly beyond any hope for returning his love, he decides to helps a young and dashing, but inarticulate acquaintance, Christian, woo Roxanne by whispering lines of poetry for him to speak to Roxanne. In this way, at least, Cyrano will be able to express to Roxanne his ardent love for her. Well, this causes Roxanne to apparently fall in love with Christian, but when he gets killed in battle, Roxanne becomes heart-broken. Cyrano’s love continues unrequited for fifteen years, until (SPOILER) at the end of his life, Cyrano reveals (accidentally) that he is the source of Christian’s eloquence, and Roxanne discovers that she had really loved Cyrano all along for the beauty of his soul which his poetic words revealed.
In all this, the term “Roxanne’s beloved” seemed to be a description of Christian, but it is really a performative description – it actually refers to the poetic soul who composed Christian’s words. Initially, Roxanne thought the referent was Christian, because, you know, he spoke the words, so she, quite understandably, thought they came from his soul. At the end of the story, it is revealed, that the words actually came from Cyrano’s soul, and so he is the actual referent of the term “Roxanne’s beloved,” his hideous nose notwithstanding.
So, the referent of a performative description is not fixed in a list of attributes, but in who or what performs an action or causes certain effects.
And it might not be known to what or even if some performative descriptions refer (whether it exists) much less what that referent is (its essence). Currently, astronomers are trying to find a Planet Nine – something, probably a planet, that is postulated to account for weird perturbations in the orbits of small objects beyond the orbit of Neptune. This thing, if it is a planet, may be rocky like Earth or a gas giant like Jupiter in very far orbit, or it may be very different, or the effects may due to some other as yet unimagined cause (or even a set of a number of objects).
Once we are able to observe the cause directly, we will know what it is – that is, its properties in a definite description – but even from when merely its existence becomes known, it will have the same referent as when there would be a description. The referent of Planet Nine would always have been the same (assuming it has a referent), even when all that was known was its existence (and that it causes trans-Neptunian object perturbations). The same thing could be said about “Dark Matter” or “Dark Energy”: some phenomena are observed and a cause is postulated, but one can’t know what the cause is until one is sure that a cause exists.
In the same way “God” as it used in Aquinas’ Five Ways is a performative description: it names a First Cause – something-or-other that explains the material universe. In this way, what is known by reason about “God” (Creator or First Cause) from the proofs of the Five Ways is, first, that the performative description has a referent, i.e., that it exists.
And anything else that Aquinas claims can be known through reason about this referent comes from inferring what must be true of it in virtue of it being the cause of the universe.
And if a person has Christian faith, as Aquinas does, she will believe that the referent for the “God” of the proof, a creator of the universe, will be the same as the God of the Christian Bible. The same would also hold for any religious monotheist – as long as they believe that their Deity is the Creator of the universe. This same rationale is why, for the vast majority of monotheists (Jews, Christians, Muslims, Brahminic Hindus, Deists, even some Buddhists) have no problem saying that they all refer to (and may even worship) the same God, though they call Him by different names. This is controversial, I know, so maybe at some other time I may explore and argue for this thesis.
One God or Many?
Finally, we can address, what is the most fundamental critique: why must the cause of the universe be a single cause? If the proofs are successful in arguing that an explanation must be found beyond the universe for the universe and its various features, why must that be a single being – a “God”?
Again, one wonders if those raising this critique have even bothered to read anything else from the Summa Theologiae from which the Five Ways come, since Aquinas addresses the unity of God in Part I, Question 11, Article 3:
First, Aquinas invokes what has come to be known as Ockham’s Razor (though, being a contemporary of William of Ockham, Aquinas would not have attributed it to the Franciscan theologian – that, and the fact that the idea has been invoked throughout the history of philosophy). Also called the Principle of Parsimony, it states that a simpler explanation is to be preferred to a more complex one. If there were many causes, Aquinas argues, it would need to be explained why they came together in a unified (and coordinated) result, namely a single universe. A single cause is better precisely because it is simpler.
Next, Aquinas gives a more substantive explanation that there is, and must be, only one God – His perfection.
“For it was shown that God comprehends in Himself the whole perfection of being. If then many gods existed, they would necessarily differ from each other. Something therefore would belong to one which did not belong to another. And if this were a privation, one of them would not be absolutely perfect; but if a perfection, one of them would be without it.”
There cannot be more than one perfect beings for, if there were, they must differ from one another in some respect. (This is an application of what has come to be known as Leibniz’s Law or the Law of Identity of Indiscernibles: if they share all the same attributes, they would be the same thing – if they don’t, they’re not both perfect). If one had a perfection the other did not, the one without it would not in fact be perfect. Hence, more than one perfect thing is impossible.
“So it is impossible for many gods to exist. Hence also the ancient philosophers, constrained as it were by truth, when they asserted an infinite principle, asserted likewise that there was only one such principle.”
And of course, Aquinas is right. Many diverse ancient Greek and Roman philosophers were monotheists: Plotinus, Marcus Aurelius, the Stoics, Aristotle, Plato, even as far back as Xenophanes who around 500 BC seems to have been the first to reason in this way: that because God is perfect, He must be one.
So, if there is anything that is truly divine, as the First Cause of the perfection we find in the universe must be, it would have to be singular. Of course, this depends on there being a First Cause of Perfection – which Aquinas proves in the Fourth Way.
Aquinas gives a final, and most definitive reason which is in fact the metaphysical basis of the other two: every creature is metaphysically complex while the creator cannot be, and so must be unique. There can only be one thing whose essence is its existence. From the very next question that follows the Five Ways, Question 3, Article 4, Aquinas says:
“Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.”
Aquinas, of course, is referring to his Third Way; so one of his proofs gives the metaphysical basis for why there can only be one First Necessary Cause of the being of the universe.
Aquinas gives a little more detail on this reasoning in Chapter 4 of a short, early work of his called On Being and Essence, that seems to be ignored by atheists and neglected by many theist apologist alike. There, he distinguishes what is meant by essence from what is meant by existence. He argues that what a thing is (its essence) is distinct from the fact that it is (its existence) since we can know the essences of both things that have existence, and of things that do not: we can know what a man is (the essence of something with existence (a real essence)) and what a phoenix is (something without existence). Since one of these essences has existence and one does not, essence is not the same existence. But an essence becomes real by having its existence caused by something other than itself.
If a thing is contingent, it might not be. And if it might not be, but is actual, then what it is, its essence, is other than what makes it to be real, its act of existing, and its act of existing is caused by something other than itself. This would apply even to temporally necessary things: things that if they exist, always exist. Such necessary things might still not be, as is clear in the case of a phoenix which never actually exists. But also if the material universe were eternal, as Aristotle believed, it also might not have been. In every case of things that might not be, both temporally limited things, like chickens or people or a universe that expanded from a Big Bang, and things not temporally limited, like a phoenix or an eternal material universe, the fact that they may or may not exist shows that what they are, their essence, is other than that they are, their existence.
And the fact that some contingent thing is real, the fact that it has an act of existing, is due to some cause outside of itself. This is true of every contingent being, even contingent beings which conceivably would always exist (as a phoenix would, if it were real, or an eternal material universe).
The only thing that can explain the actual existence of contingent beings individually and as a whole, is something that is not contingent, that is, something that is self-necessary. And the only thing that can be self-necessary is something for which what makes it real, its act of existing, is NOT other than what it is, its essence. That is, its essence, what it is, just is its act of existing.
Note that Aquinas is NOT saying that this thing CAUSES itself to be (that would be crazy, since it would have to exist before it exists in order to cause itself to exist).
Aquinas calls this the Subsisting Act of To-be Itself (in Latin, Ipsum Esse Subsistens) or Pure Act of Existing (in Latin, Actus Purus Essendi). In this way, Aquinas believes that God revealed Himself to Moses in Exodus 3:14 as “I Am Who Am” (Yahweh).
And like perfection (actually, because the act of existing is perfection), there cannot be two Subsisting Acts of To-be Itself, for if there were two, they would have to differ in some way, and one have what the other lacked. The one who had this attribute would not be its own act of existing, but the differing attribute plus an act of existing. The very purity of the Pure Act of To Be (God) make Him unique.
So the reason there cannot be more than one God is that God alone is the First Necessary Cause, the Being without a cause which makes it to be, but which ultimately causes everything that is caused to be,
– which is everything whose essence is not its own act of existing,
– which is everything other than God.
So, everything other than God is caused by the only Being that is not caused, and this can only be One and Unique.
So, to answer the objections of which God are proofs supposed to prove? Aquinas gives an answer to any atheist who cares to look:
1. The proofs show that the only thing that could cause the universe has to be a single, transcendent God.
2. In being the transcendent Cause of the universe, He is the one and only God referred to by anyone who believes, or discovers through reason, that the universe has a cause.
3. Christians, in knowing that the God of the Bible is the one transcendent cause of the universe, know that the God of the proofs refer to the God of the Bible, even though God has revealed His attributes or description more fully in and through the Bible.
And as Xenophanes observed over 2500 years ago, the God as understood through reason is nothing like the myriad divinities of mythologies, and He is not as easily dismissed as they are, despite what atheist would have you believe.
(Published February 9, 2020. Revised March 9, 2022)