Key Concept
God as God – The phrase “God as God” is basically a synonym for “God the subject.” In other words, it refers to God precisely in God’s status as an incomprehensible divine Other.
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Is there good reason to believe that God exists? And if there is good reason to believe that God exists, what does that evidence look like? Both theists and non-theists throughout intellectual history have taken an interest in this question. In the Summa Theologiae, Thomas Aquinas provides an affirmative answer to whether we have good evidence for the existence of God. He thinks if we carefully observe the world around us, we will find good evidence to think that God exists.
Thomas Aquinas (1225-1274) was an influential medieval Christian philosopher and theologian. He was especially interested in questions like “How do we know God exists?” “What is God’s nature like?” and “What is the relationship between faith and reason?” He taught at one of the most important European universities of his day, the University of Paris, where he lectured to seminarians (men who were preparing to become priests).
Our reading comes from the Summa Theologiae. Aquinas wrote this text to be an introductory textbook for his seminary students, and the text itself is divided by questions on various topics, sort of like a reference guide. Before discussing what God’s nature is like, Aquinas thinks it’s necessary to give some arguments that establish the existence of God. Aquinas gives five arguments in defense of God’s existence, and these five arguments have come to be known as Aquinas’s Five Ways, or just the Five Ways.
The kind of argument that Aquinas uses is called an inference to the best explanation. As the name of the argument form suggests, an inference to the best explanation argument begins with some observation and works toward constructing the best explanation of that observation. When you use this kind of argument, you have to think like a detective. A detective makes an observation about a crime scene and looks for the best explanation of all the evidence gathered there. This best explanation identifies both who is responsible for the crime and how the crime happened. In each of the five arguments that we will examine, Aquinas uses various observations about the universe to identify who, or what, is responsible for its existence.
Efficient Cause – An efficient cause is something that directly makes another thing exist or move. An example of this is when I kick a ball down a hill. I am the efficient cause of the ball rolling down the hill because I make it move down the hill.
Necessary Being – A being that can’t fail to exist. Its non-existence is impossible. This also means that such a being has always existed.
Contingent Being – A being that can fail to exist. Its existence is not guaranteed. This being might come to exist or it might not. For more details about necessary and contingent things, check out this article.
Infinite Regress – Begin with some fact. We begin to explain that fact by appealing to another fact, where these facts are related by either causality or dependence. To create the regress, you keep appealing to more and more facts about causality and dependence without end. For more on infinite regresses, check out this article.
Potentiality – What something has the capacity to do, but isn’t currently doing. Imagine I am sitting comfortably at my desk. Even though I’m not currently standing, I have the capacity to be standing. So, even while I’m not standing, I have the potential to stand.
Actuality – An ability or action something is currently exercising. Imagine that I am sitting comfortably at my desk, and then I stand up to take a break from reading. In this case, I am now actually standing.
In the First Way, Aquinas’s argument begins from the observation that people, animals, plants—all kinds of things—have come to exist and they are also causing other things to exist by their activity.
Aquinas is, in this argument, particularly interested in how things in the universe cause other things around them to move. He argues that all things we observe in the world are either moving in potentiality or moving in actuality. Aquinas provides the example of fire. Wood is potentially hot: it has the capacity to be hot, but it isn’t burning yet. However, once the fire sets it ablaze, that wood is actually hot. But notice that the wood can’t make itself hot: it needs the power of the spark from the fire to get it to burn. The fire exercises its power to set things ablaze—it flexes its muscles!—to move the wood from potentially hot to actually hot.
Aquinas observes this power that some things have to set other things in motion and asks, “How did all this motion get started?” Things can’t set themselves in motion: If they could, they would have to create themselves, and that’s impossible. Aquinas then argues that there has to be something from outside the chain of motion that gets it going. Here is Aquinas in his own words.
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.
In this argument, Aquinas isn’t just looking for an explanation of why this wood is hot: he wants to know why there is motion in the universe at all. So he needs the best explanation for why motion exists. With this in mind, we can understand Aquinas’s argument like this:
Premise 1: Things are in motion around us.
Premise 2: If things are in motion, then they must have been put into motion by something else that is itself in motion.
Premise 3: If things have been put into motion by something else that is itself in motion, then either (1) an infinite regress of movers explains how things got going, or (2) a First Mover explains how things got going.
Premise 4: We cannot have an infinite regress of movers that explains how things got going.
Conclusion: Therefore, there must be a First Mover who got things going.
Aquinas rules out the infinite regress of movers. In the infinite regress that Aquinas has in mind, you’d start with the fact that the wood is hot. You’d explain its heat by appealing to the fire that lit it. And then you’d appeal to how that fire got hot. And then you’d appeal to some fact about what made the fire get hot. In an infinite regress like this, you never stop appealing to further facts about causality in order to explain how the wood is now hot. But Aquinas thinks such an explanation fails to tell us why there is motion in the universe at all: it just describes the motion that does exist in this particular chain of causality. But remember that Aquinas’s argument is trying to identify why there is motion at all and not just one instance of motion. That failure to explain is why Aquinas rejects the infinite regress of movers and claims that the First Mover is the best explanation of why there is motion at all. The First Mover, who has always been in motion, got everything started.
In the Second Way, Aquinas makes a second argument that begins from the observation that there is efficient causality in the world: stuff causes other stuff to happen. As in the previous Way, Aquinas wants to know, how did all this causality get started in the first place? Aquinas argues as follows.
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.
As in the First Way, Aquinas is interested in this second argument to explain an observation: Why do we see efficient causality in the universe? Let’s break down Aquinas’s argument to understand why he thinks that God is the best explanation of efficient causality in the universe. He begins with an observation that efficient causes are also themselves caused:
Premise 1: There are many efficient causes in the world which are the effects of prior efficient causes.
Premise 2: If there are many efficient causes in the world which are the effects of prior efficient causes, either (1) an efficient cause is the efficient cause of itself, (2) there is an infinite regress of causes that explains all efficient causality, or (3) there exists a First Cause.
Premise 3: Nothing can be the efficient cause of itself.
Premise 4: An infinite regress of efficient causes cannot explain all efficient causality because all the efficient causes together still don’t explain why there is efficient causality in the first place.
Conclusion: Therefore, there must exist a First Cause.
In Premise 2, Aquinas gives us three options for how to explain why there is causality in the world, and then as he moves through the argument, he rules out two of those options. He rules out the first option, that something could cause itself. It’s impossible to exist before you exist in order to bring yourself into existence! He also rules out the second option, that there is an infinite regress of efficient causes. Aquinas’s explanation of this infinite regress option is a bit different from what we saw in the first argument. Aquinas wants us to see that, if there’s no ultimate first cause, we cannot explain the effects of efficient causality at all.
In efficient causality, effects depend upon their causes for their existence, and so on up the chain. The movement of the ball depends upon the movement of my foot which is dependent upon my intention to kick the ball, and so on. But notice that if we have no First Cause, we have no way to explain at least one fact in that causal chain. The causal chain has to bottom out somewhere, otherwise we just keep adding items to the regress. But notice that adding items to the regress doesn’t tell you the complete story of how there is motion at all. And that failure to complete the explanation is why Aquinas rejects the infinite regress in this argument.
Once Aquinas rejects the infinite regress, and denies that something can cause itself, Aquinas claims that the option that best explains the existence of all this efficient causality is God.
In this argument, Aquinas notices that there are beings that are dependent on other beings for their existence. Think about the fact that you are dependent on your parents for your existence. Without them, there is no you. But this dependence means that it’s quite possible that you would not have existed. Think about if your parents never met! Such dependence requires an explanation as to why you in fact do exist: there’s a good reason you’re here. And so, Aquinas thinks, the world is full of beings that are dependent in this way. There has to be some explanation, some good reason, why they exist as opposed to not existing. In our text, Aquinas argues as follows.
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 as God.
Think about the movie It’s a Wonderful Life: George Bailey wonders what it would be like if he’d never been born, and he gets to see a possible version of a world where he doesn’t exist. Aquinas’s idea here is similar. It’s quite possible that you might never have existed. But Aquinas goes farther than George Bailey in the thought experiment: Aquinas thinks that, because you quite possibly could have never existed, something greater than you must explain why you are here. Someone has to bring you into existence, and who they are and what they have done explains your existence. But if all we have in the universe are beings who are dependent on other beings for existence, it’s quite possible, Aquinas thinks, that nothing might have existed at all. That’s because, for all beings whose existence could have been or could have not been, there’s a time at which those beings don’t exist. So something that has always existed, a necessary being, has to bring the non-existent into being. Here’s one way to think of Aquinas’s argument in the Third Way:
Premise 1: The universe is full of possible beings whose existence requires that some necessary being created them.
Premise 2: If the universe is full of possible beings whose existence requires that some necessary being created them, then some necessary being exists.
Premise 3: If some necessary being exists, then either (1) an infinite regress of created necessary beings explains the existence of these possible beings, or (2) the Necessary Being exists and explains the existence of the possible beings.
Premise 4: An infinite regress of created necessary beings cannot explain the existence of the possible beings since these necessary beings are themselves created.
Conclusion: The Necessary Being exists and explains the existence of the possible beings.
One possibility for explaining the universe that is offered in Premise 3 is that you could have an infinite regress of created necessary beings to explain how the merely possible or contingent beings came to be. The thought goes like this: created necessary beings possess the necessity to explain the creation of contingent things. So why do we need to appeal to one entity called the Necessary Being instead of many necessary beings? The problem with many necessary beings is that, if a necessary being is created, that means it received its necessity from something else. If this created necessary being received its necessity from something else, it requires a similar kind of explanation as the contingent being for why it is a necessary being. In other words, there exists some being outside of the created necessary being that explains why this being is a necessary being. To fully explain the existence of all created things, whether they are necessary beings or contingent beings, there must exist a being who is not created and who possesses their necessity independently of anything else. Enter the Necessary Being. This Being has always been and has never not been. And this Being is self-sufficient, meaning that it doesn’t receive its necessity from outside of itself. In other words, you don’t get a full explanation of why something created exists by appealing to beings that are themselves created. You need something outside of the created universe to explain its existence.
In this argument, Aquinas starts with the observation that some things have more or less of some attribute. Aquinas gives the example of heat. Some things are hotter than others. An oven baking cookies at 350° and the sun are both hot, but the sun is definitely hotter than the baking oven. Nevertheless, we properly call both the oven and the sun hot because they share in the nature of heat, and what it means to be hot. Aquinas takes this idea and applies it to being and to goodness.
Among beings there are some more and some less good, true, 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…
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.
Aquinas thinks that all existing things possess being and goodness but in varying degrees. For example, Aquinas thinks that a human being is the kind of thing that uses their reason to think well and to act well, especially when it comes to moral excellence. So Aquinas thinks that a good human being is a human being that uses their reason well and acts with moral excellence. But notice that among all the human beings that have existed, some human beings have been better at reasoning well than others, and some have been better at moral excellence than others. In other words, while all human beings possess some goodness, all human beings possess different degrees of goodness.
But notice that if you have degrees of goodness like this, there has to be an ultimate standard of perfection against which these degrees of goodness are compared. And Aquinas argues, this ultimate standard of goodness is what we could call the Perfect Being.
We’ve seen four ways that Aquinas uses observations about the natural world to argue for the existence of something that best explains those observations. But do you think that Aquinas has tapped out all the sources of evidence? Are there observations about the world that Aquinas doesn’t use that you think are good evidence for God’s existence? Think about what you might observe from human nature or about our appreciation of beauty. Do any of those things provide evidence that there is some kind of deity? Could you make an argument based on that evidence?
In this argument, Aquinas observes that there is a directedness in the natural world. But what does Aquinas mean by directedness? He is referring to the idea that, in the natural world, things aim towards a specific goal, and they especially aim towards what makes them flourish. Here’s what Aquinas argues.
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.
We make plans and do things because we think those things will be good for us. But Aquinas notices that it seems that even creatures that have no intelligence, such as plants, also do this. When I was in elementary school, I performed an experiment for science class. I planted a bean in damp soil and left it in a dark cabinet. I checked on the bean after about a week, and it had sprouted towards what little light was in the cabinet. How did the little bean sprout know to do that? To have an aim or a plan requires that something is thinking and reasoning. But plants don’t think and reason! And yet that bean sprout grew towards sunlight. Aquinas says that the best explanation of how unintelligent plants can do this is that someone or something is doing that reasoning and planning on their behalf, directing them towards what they need to flourish. And this director just is the intelligent being we call God.
In these Five Ways, Aquinas makes five observations about the universe, and from those observations, argues for the existence of God. But you might imagine someone objecting like this: “Isn’t Aquinas a Christian philosopher? These Five Ways don’t establish the truth of Christianity. So how are they helpful in thinking about the Christian faith?”
I think Aquinas might agree with this objection. But notice that this is not a problem for him as far as the Five Ways go.
Remember that Aquinas argues that (1) there is a First Mover, First Cause, Necessary Being, Perfect Being and Intelligent Designer and that (2) we equate these with something we’d call a deity. Because Aquinas is a systematic philosopher, he’s going to start from the very beginning in this textbook for his seminarians. In the Summa Theologiae, Aquinas begins with questions about God’s existence in Part I and ends with questions about Christ, sacraments and sin in Part III. The arguments in the Five Ways lay the foundation for the arguments about Christian doctrine at the end. Or, in other words, someone has to have good reason to believe that God exists before they would believe that Christianity is true. So, the Five Ways fit into this larger work as the starting point for the seminary student who is pondering the truth of the Christian faith.
Let’s return to the question posed at the beginning of this article: Do we have good evidence that God exists? Aquinas’s answer to this question is yes. Motion, efficient causality, intelligent design, the existence of contingent beings, and the existence of perfection provide Aquinas with evidence from which to argue that God exists. But we don’t want to take Aquinas’s arguments for granted. Part of doing good philosophy is thinking through the arguments that are presented to you. So what do you think of Aquinas’s arguments? Which of them, if any, did you find convincing? Why were they convincing? Pondering these questions will put you well on your way to discovering whether there is good evidence for God’s existence.
For more on the context and big ideas behind Aquinas’s Five Ways, check out this video:
If you want to dive deeper into the thought of Aquinas on God’s existence as well as God’s nature, you’ll want to check out the following two resources: The first resource is a text by Frederick Copleston, called “A History of Philosophy: Volume II: Medieval Philosophy” (New York: Doubleday, 1993). The second resource is an online article by Shawn Floyd, “Aquinas: Philosophical Theology”
If you would like to know more about Aquinas’s life and works, check out Ralph McInerny and John O’Callaghan’s article “Saint Thomas Aquinas” and Robert Pasnau’s article “Thomas Aquinas.”
Has this article made you curious whether there are other philosophical arguments for God’s existence? Well, you’re in luck—Aquinas is one of many philosophers of religion (those philosophers who work on questions relating to God’s existence and nature) who have pondered these questions. Check out Chad Meister’s article entitled “Philosophy of Religion” to learn more about the different kinds of arguments philosophers have offered in defense of God’s existence.
This work has been adapted from Summa Theologiae, translated by the Fathers of the English Dominican Province. This work is in the public domain. I am also indebted to correspondence between myself and Drs. Edward Martin and David Beck for my interpretation of Aquinas on certain points. The work of Fredrick Copleston was especially helpful in the writing of this article. (Frederick Copeston. A History of Philosophy: Volume II: Medieval Philosophy. New York: Doubleday, 1993.) Sean Floyd’s article “Aquinas: Philosophical Theology” was also influential in the construction of multiple parts of this article, especially the discussion of the Second Way and the Objection sections. All images were created using Midjourney and are the property of the Philosophy Teaching Library.
McCarty, Emily. 2024. “Five Ways to God: Thomas Aquinas’s Summa Theologiae Part I, Question 2, Article 3.” The Philosophy Teaching Library. Edited by Robert Weston Siscoe, <https://philolibrary.crc.nd.edu/article/five-ways-to-god/>.
God as God – The phrase “God as God” is basically a synonym for “God the subject.” In other words, it refers to God precisely in God’s status as an incomprehensible divine Other.
Incarnation – The Christian doctrine of the incarnation is the notion that the word of God became fully human in the person of Jesus of Nazareth. It is closely associated with the doctrine of the trinity, which asserts that God the Father, God the Son (Jesus as the word made flesh), and God the Holy Spirit are one God.
Religious Fanaticism – In Feuerbach’s use of the term, a religious fanatic is someone who is unwaveringly faithful to God as an utterly mysterious superhuman being. They subordinate other things—especially the love of other humans—to submission before this divine other.
God the Subject – When Feuerbach refers to God as a subject, he is referring to the commonplace religious belief that God is a being who has various attributes, like a loving nature.
Faith Separates Man From God – Faith separates God from man in this sense: it treats God as a mysterious other, a being radically distinct from us.
Faith – Belief in and fidelity to a transcendent divine subject like God.
Orthodoxy – Orthodoxy refers to “right belief,” and it is concern with identifying heresies and ensuring that people believe and practice correctly.
Indirect Form of Self-Knowledge – Feuerbach’s view is that religious belief is a naive way of relating to our human nature and its perfections. It is naive or childlike because it treats these as external realities that belong to God. He believes a mature and contemplative person realizes these don’t belong to God, but rather to our species, abstractly conceived.
Above the Individual Man – The human perfections are “above the individual” insofar as no particular individual ever perfectly realizes them. They are abstractions.
Divine Trinity – Feuerbach is having fun here. He is using the theological phrasing of the Trinity to talk about human perfections. In calling reason, love, and freedom of the will “divine,” he means they are absolutely good; they are activities whose goodness is intrinsic to their practice or exercise. This isn’t a novel philosophical view. For example, Immanuel Kant argued that autonomy or a good will is the only thing which is unconditionally good.
Perfections – The end to which a faculty or power is ordered. For example, omniscience would be the perfection of the intellect. Traditionally, God is said to possess all perfections.
Love – When Feuerbach writes about love, he is referring to unconditional concern for others and the desire for fellowship with them. He is here asserting that love, understood in this sense, is the perfect activity of the affective faculty. In other words, our feelings and passions are fully actualized and engaged in an intrinsically valuable activity when we genuinely love others.
Infinite – The infinite is whatever can be understood as unbounded or unlimited. Human nature in the abstract is unbounded and unlimited. It is only bounded or limited in its concrete form as it is realized by particular material individuals.
Higher Consciousness – The sort of consciousness that mature human beings possess, but which other animals do not. It is “higher” than animal consciousness because it involves thinking abstractly about the form or essence of things.
Science – Feuerbach uses the term science in its classical sense, meaning systematically organized knowledge. Any body of knowledge founded on an understanding of first principles and the essences of things is a science in this sense.
Popular Sovereignty – The view that a government’s authority to rule comes from the people, making a ruler subject to the will of their citizens.
The Divine Right of Kings – The theory that kings are chosen by God and thus that political revolt is a rebellion against the will of God.
Synthesis – The prefix ‘syn-’ means “together,” so a synthesis “brings together” or combines elements of both a thesis and its antithesis.
Antithesis– An antithesis is the contradiction of a thesis. For example, internationalism could be understood as the antithesis of nationalism.
Thesis – In Hegelian terms, a thesis can be understood as a position or theory. Examples include any of the “-isms” that we discuss in science, history, and philosophy, such as Darwinism, capitalism, nationalism, etc.
Progressor’s Temptation – a unique temptation for those making progress in which pride impedes their further progress and leads to backsliding.
Progressors – those who are not yet expert Stoic practitioners, but who are also aware of the fact that they must change their lives in that direction. They are working on making progress.
Intellectualism – the philosophical view that our motivations and emotions are all judgments. The reason why you do something, your motivation, is because you believe it’s the right thing to do. The reason why you feel good or bad about something, an emotion, is because you believe that something good or bad happened to you.
Duties – acts of service, obedience, and respect that we owe to each other. The duties we owe to each other depend on what kind of relationship we have.
Askeses – exercises of Stoic thought and practice that make the lessons and habits of Stoic philosophy second-nature for Stoic practitioners.
Externals – things that are not under our control but that are all-too-easily confused with things that should be important to us, like wealth, status, and pleasure. Too many people believe externals like these are necessary for the good life, and the Stoic path is to focus not on these things but rather what is up to us.
The Fundamental Division – the division between things that are under our direct control and those that are not. The important lesson is to care only about the things we can control.
The Greatest Happiness Principle – A principle which says that actions are right insofar as they promote happiness and wrong insofar as they promote unhappiness
Higher and Lower Pleasures – Types of pleasures that differ in terms of their quality. Things like food and drugs create lower pleasures. Things like intellectual pursuits and doing the right thing create higher forms of pleasure.