Brandon Vogt

Can we know God exists?

St. Thomas
Today marks the feast of St. Thomas Aquinas, perhaps the greatest thinker in Church history. The Dominican prodigy is best known for his two massive “Summas”, the Summa Theologica and the Summa Contra Gentiles, along with a wealth of other writings on Scripture, theology, and philosophy. Pope Benedict XVI recently noted St. Thomas’ influence on the Church:

“It is not surprising that, after St. Augustine, among the writers mentioned in the Catechism of the Catholic Church, St. Thomas is quoted more than any other—some 61 times! He was also called the Doctor Angelicus, perhaps because of his virtues, in particular the loftiness of his thought and purity of life…
In short, Thomas Aquinas showed there is a natural harmony between Christian faith and reason. And this was the great work of Thomas, who in that moment of encounter between two cultures—that moment in which it seemed that faith should surrender before reason — showed that they go together, that what seemed to be reason incompatible with faith was not reason, and what seemed to be faith was not faith, in so far as it was opposed to true rationality; thus he created a new synthesis, which shaped the culture of the following centuries.”

For a brief introduction to St. Thomas and his work, check out this video by one of his most devoted disciples, Fr. Robert Barron:

If you’d like do go even deeper, here are my favorite four books on St. Thomas:



Thanks to Kevin Knight at New Advent, who has digitized St. Thomas’ entire Summa Theologica, below you’ll find an excerpt from the First Part which outlines St. Thomas’ famous five proofs for the existence of God.
It should be noted that these aren’t proofs for God in the mathematical or scientific sense. Instead these are arguments appealing to the logical evidence for God. Also, St. Thomas uses philosophical terms like cause, necessity, and existence which for him carry very precise technical meanings which are often different than how we use those words today. Thankfully, Kevin has linked many of these terms to their entries in the Catholic Encyclopedia so if you come across one you’re unfamiliar with, click on the link to understand it better.
As per his usual style, St. Thomas begins with objections to his position. In this case, the two objections are the existence of evil and Occam’s Razor. Next he appeals to an authority who disagrees with the objections (often the Bible, Aristotle, or St. Augustine), then he explores a possible answer, and then finally he refutes the original objections. The selection below will not only answer the question, “can we know God exists?”, but will also give you a taste of St. Thomas’ characteristic style.


Article 3. Whether God exists?

Objection 1. It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word “God” means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.

Objection 2. Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God’s existence.

On the contrary, It is said in the person of God: “I am Who am.” (Exodus 3:14)

I answer that, The existence of God can be proved in five ways.

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

St. ThomasThe second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of asGod.

The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less goodtrue, noble and the like. But “more” and “less” are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum 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.

The fifth way is taken from the governance of the world. We see that 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. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

Reply to Objection 1. As Augustine says (Enchiridion xi): “Since God is the highest good, He would not allow any evil to exist in His works, unless His omnipotence and goodness were such as to bring good even out of evil.” This is part of the infinite goodness of God, that He should allow evil to exist, and out of it produce good.

Reply to Objection 2. Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article.

What do think of St. Thomas’ arguments?


  • Ken

    When you know yourself you will know God.

  • Csjoycejd1

    I hope we are not all talking past each other here. As always with philosophical discussion, terms should be defined at the outset. “Scientific” in the narrow and most commonly used sense only includes the empiriometric, ie. physical. However, in its broadest sense, “scientific” includes philosophical reasoning and logic. What I am concerned about, and I believe others would echo this sentiment, is the notion that somehow Aquinas’ arguments or ways or proofs or whatever you want to call them FAIL to definitively PROVE God’s existence. They do not fail. They succeed.

    • I agree. They do prove God’s existence, but not in the same way as, in the empirical sciences, one “disproves” a scientific thesis. I think we’re all in basic agreement about that fact.

      The reason I’m hesitant to use “proof” to describe each of St. Thomas’ arguments is that the modern New Atheist bandies about, clamoring for “proof” or “evidence” for God’s existence, when what he really desires is “empirical evidence.” Since God, by nature, cannot be proved empirically, I and many others choose to refer to St. Thomas’ claims as “ways” or “arguments.” It doesn’t diminish them or deny their efficacy, but it does distinguish them from empirical proofs.

      • Csjoycejd1

        Very good

  • WRBaker

    I taught the general concepts of the Quinque Viae to my 8th graders and they “got it.”

  • Schlukeif

    I liked your post. I don’t know what Aquinas’ intentions were for the proofs, but we should be very careful about throwing these proofs around at non-Catholics or non-Christians under the pretense of evangelization. More often than not the outcome will be an inflated sense of self-righteousness on our part while the other person will be even further from the faith than before.

    Contingency, especially human contingency, is just as valuable a thing to be proved. Demonstrating the fact of human contingency is all the more important in a secular humanist cultural environment that affirms individual autonomy and individual liberty as self-evident and unquestionable truths. Science can be our valuable ally in proving contingency. Biologists have proven rather conclusively that our existence is made possible by the profoundly complex cooperative relationship of the trillions of cells that comprise our body. In addition to our own cells trillions of bacteria, which outnumber our own cells 10-1, also “call” the human body home. If these cells assert their liberty and autonomy (cancer), we learn just how contingent we are.

    The human progress made possible by science is also a stunning example of human contingency. Science could be characterized as human beings collectively trying to conform (or convert) our ideas and behavior to objects and phenomena in our world. In the case of particle accelerators and many other scientific endeavors this process can be profoundly expensive and time-consuming. Once we’ve used science to undermine our liberty then we use engineering to undermine our autonomy. Humanity’s profound and growing dependence on technology undermines as much as anything else enlightenment illusions of human autonomy. Science and technology are often cited as examples of human beings gaining power over nature, but the process is more complex than that. In order to access the power of science and technology we must first sacrifice our intellectual independence and self-sufficiency. Rather than being opposed to Christianity, science provides a powerful metaphor of the spiritual life and illustrates the truth of the beatitude: The meek shall inherit the earth.

  • Ben

    Yasha, maybe nothing would delight St Thomas more than the rational battle of ideas over what some might think ‘pedantic’ distinctions! (Though there has perhaps been more rancour than necessary.)

    An enlightening quote comes from Blessed John Paul II: ‘…proofs of God’s existence are not lacking. These have been elaborated by thinkers under the form of philosophical demonstrations in the sense of rigorous logical deductions…we are not speaking of proofs in the sense implied by the experimental sciences. Scientific proofs in the modern sense of the word are valid only for things perceptible to the senses.’ (General Audience, July 10, 1985)

    Metaphysical philosophical proofs, such as those for God’s existence, are different from experimental scientific proofs, and actually surpass the latter in certainty, being ‘rigorous logical deductions’. (Scientific conclusions generally are tentative to at least some small degree.)

    In respect of their rigor and certainty, metaphysical proofs are similar to mathematical proofs. However, they shouldn’t really be described as ‘mathematical’ (in the way the word is typically used) since they aren’t dealing with numbers (unless anything ‘logical’ is called ‘mathematical’).

    Also, Aquinas’ proofs do rely (unlike mathematics) on at least one empirical existential premise known by sense experience of reality (i.e. ‘Something exists [which needs an explanation]’). (This is what keeps his proofs in touch with the real world.)

    So Brandon was technically correct to say these proofs weren’t scientific or mathematical. When properly understood, with sufficient background and elaboration, the proofs are fully demonstrative. But the ‘understanding’ part can be a difficult road!

    • Thanks for this excellent comment, Ben! I’m glad Pope John Paul II echoed my original qualification, which was meant to express something similar:

      “…we are not speaking of proofs in the sense implied by the experimental sciences. Scientific proofs in the modern sense of the word are valid only for things perceptible to the senses.”

  • Yasha Renner

    Stop bickering on the Saint’s Feast Day!

  • It is, by the way, de fide from Vatican I that God is knowable with certainty by the light of human reason: “If anyone says that the one, true God, our creator and lord, cannot be known with certainty from the things that have been made, by the natural light of human reason: let him be anathema.” (Dei Filius, Canons, II.1)

    One can argue, if one likes, that this was not St. Thomas’s intention, but one cannot argue that it is not possible, and I don’t know of better contenders than St. Thomas’s proofs.

  • Professor

    Fr. Geoff makes a series of awesome points. I don’t think either of us means to be a pedant, Mr. Rimmer. Mr. Vogt is obviously a great guy. On the other hand, it is a very serious error to assert that St. Thomas did not intend to PROVE the existence of God, in exactly the same way that Euclid proved the Pythagorean theorem in Elements 1:47. No pedantry here. It’s just a very serious point–proving the existence of God through reason alone was St. Thomas’ precise objective here.

  • With respect to my comments on mathematical certainty, might I suggest Morris Kline’s _Mathematics: The Loss of Certainty_? It’s a fairly easy read, especially for the mathematically inclined, and it shows (I think largely unintentionally) how great a mistake Descartes made in setting up mathematical “proof” as a measure of certainty different from other measures.

  • David Naas

    (Who Would Have Thought…)
    Saint Thomas believed FAITH was necessary, not mere Reason. Reason edifies Faith.
    (Watch out for mystics, they’ll get you every time.)
    And, in celebration, I offer this…
    Ny five volume edition of the Summa is on the way… have to do Something in retirement… 🙂

  • Brandon,

    It seems as though your comments section is quickly becoming infested with pedants. You respond with more grace than I would, or will.

    If Aquinas really thought that his proofs were proofs in the mathematical sense, then Aquinas was an idiot. He argued in the previous question that God’s existence is not self-evident. If he thought the proofs were meant to establish God mathematically, he’d disagree in his Question 3 with his answer to his Question 2.

    Since Aquinas is not an idiot, I must conclude that his use of the term “proof” was not meant in the mathematical sense.

    Excellent article, Brandon. Thank you.

    • Csjoycejd1

      you are wrong. Many things are scientifically proven through demonstrative argument that are not self-evident. God’s existense is one one of these things. Aquinas certainly was no idiot.

      • Exactly. Demonstrated scientifically. Not proved in the mathematical sense.

        If you agree with my comment, and say that I’m wrong, what does that make you?

        • Csjoycejd1

          So you agree that Aquinas’ arguments prove God’s existence? If so, you are correct, and I misunderstood your comment. That also means that you disagree with Mr. Voigts original comments in the article, as I understand them. It seems he has modified his thought in the comments’ section.

    • “Demonstrable” and “self-evident” are not synonyms. Fermat’s Last Theorem has been shown to be true, yet even those few who understand the proof completely would never argue that it is self-evident.

      Self-evidence is where the axioms come from. In saying that it is not self-evident that God exists, St. Thomas is saying that it is not axiomatic and therefore is in need of demonstration from premises that are self-evident.

  • Reading in the philosophy of mathematics will surprise you. Descartes wanted to bring the certainty of all thought to mathematics; the acid of skepticism ate away at mathematics to the point where mathematicians are not sure they are doing anything but word games. All of this meaning that an attempt to divide St. Thomas from mathematical and scientific reasoning is doomed, not from his side but theirs. It turns out that they also rely on “premises and logic that can be questioned,” and they have no idea where to turn for the foundations they need.

  • Clementius

    What do you mean by the greatest “thinker”? St. Thomas was and is the greatest theologian. A “thinker” opines; a theologian reasons. There’s a difference. His reasoning actually prove the existence of God. A thinker almost always comes to the conclusion that there is no God. He is not capable of reasoning.

    • Clementius, by “thinker” I mean, unsurprisingly, one who thinks. To think *is* to reason. I agree St. Thomas was and is the greatest theologian in the Church (outside of, perhaps, St. Paul and St. Augustine.) But he was also an unmatched philosopher.

      You say, “a thinker almost always comes to the conclusion that there is no God” which is not just false, it’s egregiously false. The overwhelming majority of thinkers throughout history, along with the present, have reasoned their way to God.

      Your last claim–“[St. Thomas] is not capable of reasoning”–is too silly to deserve a response.

      • Clementius

        Brandon, I’m surprised that you deliberately read between the lines. Note that the “he” refers to the thinker, following the sentence just previous to it (i.e., the “thinker” and not St. Thomas. Read and understand, silly. Tolle lege!)

        As for comparison, St. Augustine and St. Thomas are equal in their orthodoxy, but differ in their philosophical principles: St. Augustine is Platonic; St.Thomas is Aristotelian.

        There is no room for opinion when it comes to discussing the purity of the Catholic faith.

  • Professor

    Brandon, your analysis of St. Thomas is severely flawed and profoundly misleading. The whole point of St. Thomas’s proofs is to “demonstrate,” that is to say “definitely prove” God’s existence–NOT merely to offer probable arguments for it! One’s subjective inability to grasp the proofs should not obscure the undeniable fact that St. Thomas INTENDED them TO BE DEFINITIVE PROOFS. Obviously one needs some philosophical background to appreciate the demonstrative nature of these proofs–but you CANNOT say that St. Thomas was not attempting to “definitively verify” God’s existence.

    • Professor, I understand your critique and mostly agree. But my point was that by “proof” St. Thomas did not mean what we moderns mean by “proof.” Instead of the tight, non-syllogistic style of mathematical proofs, St. Thomas’ “five ways” still rely on premises and logic that can be questioned (albeit, we would agree, wrongly.) If St. Thomas’ arguments were really “proofs” in the modern sense then *everybody* would quickly agree, just as everyone agrees that “the sum of an even number and an odd number is always odd.”

      • Professor

        I don’t think you really understand what I meant, my friend. St. Thomas absolutely did mean “proof” in the sense of mathematical proof. You can argue that his proofs are unsuccessful, but you cannot argue that he meant “proof” in some weaker sense of “probable argument” or something like that. Just because his proofs are not accessible to those who lack philosophical training does not mean that he did not intend them to be proofs in the strict sense. That was the whole point of what he was up to.

        • Fair enough, Professor. I just think St. Thomas’ “ways” are nevertheless distinct from proofs in other areas like mathematics and science. That doesn’t make the “ways” any less airtight or persuasive than other proofs, only different.

          But for the sake of discussion, I’ll cede the point and agree that St. Thomas was out to “prove” God if “proof” is properly understood.

          • Geoffrey Miller

            As a practicing mathematician, Brandon, St. Thomas Aquinas “ways” are indeed in the same category as mathematical proofs. They employ analytic reasoning and use first principles or axioms.

            I have no idea what you’re talking about when you refer to scientific proof. The sciences, even the “hard” ones like physics, do not use “proof” as it is understood by mathematicians. They argue by using inductive reasoning and statistical inference.

            Please modify your post. Overall, it was good, but as a professional practitioner of both mathematics and physics, and also as an educator in those fields, I really don’t like to see people perpetuating misunderstandings about them.

            Thank you,
            Geoffrey Miller, M.S., Doctoral Student at Texas State University-San Marcos in Mathematics Education

          • Geoffrey, thanks for the comment. I’m not terribly interested in continuing this discussion, which has already gone too long and is really peripheral to the goal of my original post, but it’s worth pointing this out: despite your assertion, philosophical proofs *are* definitively different than mathematical and physical proofs.

            (And while I appreciate your impressive credentials, they don’t change that fact.)

            Though you claim to be a “practitioner in…physics”, you surprisingly say:

            “I have no idea what you’re talking about when you refer to scientific proof.”

            Scientific proofs are fundamental and pervasive in all branches of empirical science. While it’s true that most scientific proofs are ‘negative’ proofs in that they disprove erroneous theories rather than verify positive ones, they are still proofs in the proper sense.

            Even Pope John Paul II noted the existence of scientific proofs and their distinction from philosophical proofs, as noted in the quote which Ben posted below:

            “we are not speaking of proofs in the sense implied by the experimental sciences. Scientific proofs in the modern sense of the word are valid only for things perceptible to the senses.”

            This qualification, however, verifies my original point: scientific proofs, like mathematical proofs, are of a different nature than the philosophical proofs St. Thomas appeals to.

            This is why “proof” language is often misleading to the average layperson and why St. Thomas’ arguments have been referred to as “ways” or “arguments” down through history.

            I won’t respond further, but if you still adamantly disagree I encourage you to write a post on your own blog in response.

      • Imrahil

        No. Reality is the other way round:
        People would dispute 2+2=4 just as much as God’s existence, were it (felt as) a matter of morality and to live your life according to.

        Although the proof is indeed maybe not as direct. Especially that the recourse to the infinite really can be excluded (to St. Thomas obviously a self-evidency), and the concept of the cause in proof 4. If infinite recourse can be excluded… well then proof 1 holds. Proof 3 holds anyway.

  • Rob B.

    The Dumb Ox rocks! 🙂

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