Does God Exist? Aquinas’ First Two Ways

Thomas Aquinas famously gave five arguments for the existence of God in his Summa Theologica. Today I am going to discuss the first two:

  1. The argument from motion.
  2. The argument from efficient causality.

The strengths of Aquinas’ arguments, in my opinion, are:

  1. They are not based upon scientific premises, so scientific objections are void.
  2. If the premises are true, the conclusions necessarily follow. They are not probably true, they must be true.

A major weakness of Aquinas’ arguments, if this can even be considered a weakness, is that we are far removed from his context and therefore some of the terms he uses are easily misunderstood. In fact, many of the objections to Aquinas’ arguments are based on a misunderstanding of his terms. But this can easily be remedied by a proper explanation.

The Argument from Motion

It is evident to our senses that some things are in motion. Since motion is nothing more than the reduction of a potentiality to actuality, nothing can set itself in motion, but must be put in motion by something else which is already in act. That which is already in motion must itself be put in motion, and so forth. This cannot go on infinitely for there would be no first mover and consequently no intermediary or final movers. Therefore, it is necessary to arrive at a First Mover (Summa. Part 1, Question 2, Article 3).

First, notice that Aquinas does not say “all things are in motion.” This is important to note because some have tried to object by saying “If everything is in motion, what put God in motion.” This is of course absurd because Aquinas never said that everything was in motion. Not to mention, the very point of this argument is to show that the First Mover is not in motion. It has not potentials to actualize. It is Pure Act.

Secondly, Aquinas’ definition of “motion” is a bit different than what the modern reader thinks of. He does not here simply mean local motion like the kind you observe on the highway. Rather he means something like change. For Aquinas, motion is nothing more than the act of moving from potentiality to actuality. The acorn is potential an oak. One day it will actually be an oak. So when he says “somethings are in motion” he is saying “somethings are moving from potentiality to actuality”. The second premise is that they cannot move themselves from potentiality to actuality, but must be acted upon.

Next, consider the kind of causal series Aquinas has in mind. He is not thinking along the lines that some have asserted. He is not arguing that if you follow the chain of causes and effects backward in time, you’ll arrive at a being that set everything in motion. Aquinas’ argument is not based on a universe that began. He isn’t arguing for a God that set everything in motion a long time ago and we observe the effects today. He may have believed this, but that isn’t what he is arguing for here. No, his argument is much stronger.

Aquinas has in mind a causal series ordered per se, or essentially. In the Summa, he gives the example of a hand and a staff. Dr. Edward Feser provides a great example in which he adds to Aquinas’ illustration. Consider a hand which is moving a staff, which is moving a stone, which is moving a leaf. In this illustration, all of the components are moving simultaneously. It isn’t that the hand moves at one point in time, the staff moves at another, and so forth. No, all move at the same time. Because of this, all of the lower components are completely dependent on the hand, which in this illustration is the first mover. In reality, the hand is being moved by muscles in the arm, which are being moved by neurons in the brain, and so forth. But this is merely an illustration to demonstrate the kind of causal series Aquinas has in mind. In causal series ordered per se, there must be a first mover. To not have a first mover is not to move at all because all of the lower elements are completely dependent on the first mover for their own motion. To not have a first mover is to not have a causal series ordered per se.

Whatever else God is, He is the First Mover.

The Argument from Efficient Causality

In the world of senses there is an order of efficient causes. A thing cannot be the efficient cause of itself, for it would be prior to itself, and that is impossible. In efficient causes it is impossible to go to infinity, therfore it is necessary to arrive at a First Cause.

Again, Aquinas does not say that “everything has a cause” therefore the objection that “If everything has a cause, what caused God?” is simply void.

As with the first argument, this argument is also not dependent on a universe that had a beginning. As far as Aquinas is concerned, the universe may be past-eternal, it doesn’t matter. He is not arguing for a God that caused everything to come into existence a long time ago. He is arguing for a God that causes things to exist right here and now. As in the first argument, he has in mind a causal series ordered per se which I explained above.

The difference between the first and and second arguments is that in the first argument, Aquinas is explaining how anything here and now can move, or be actualized from potency, by positing a First Mover. In the second argument, Aquinas is explaining how anything can exist at all here and now by positing a First Cause.


Aquinas’ arguments for the existence of God seem outdated to many. When examined, this is usually because of a misunderstanding of what Aquinas is actually saying, or because of the flaws of modern philosophy. However, Aquinas’ arguments have been successfully defended by contemporary philosophers. For further reading, I highly recommend Edward Feser’s Aquinas: A Beginner’s Guide.  Of course, you can read straight from the horse’s mouth and read Aquinas’ Summa Theologica.


6 thoughts on “Does God Exist? Aquinas’ First Two Ways

  1. This is indeed weighty stuff! I am thankful to God for all the great thinkers that He has sent through the ages even until modern times to teach us to defend the faith through apologetics and reason. This is a great example of that. Thanks for sharing!


  2. I do see a few problems with your explanation which I will try to discuss.

    First, the claim that Aquinas’ arguments are not based on science, there fore any scientific rebuttal is impossible. Well, that is certainly handy. But Aquinas DOES make scientific arguments. The laws of motion and causality do not exist in thin air (unless you are claiming that all Aquinas was doing was performing a mental exercise). Motion cannot occur without something being moved. Which brings us to the science of physics and the physical world.. Otherwise, what is the motion in reference to? Nothing?

    The same can be applied to the causation argument. SOMETHING, in the real world is being “caused” to happen. Otherwise, all we have is Aquinas exercising some notions which have no place in the physical world, are are therefore irrelevant to a god at all. So, your first suggestion that Aquinas is not involved in making a case for the explanation of the physical (scientific) world does not seem to hold water (real or imaginary water).

    After all, Aquinas argues for the EXISTENCE of god in the physical world. To deny the idea that those arguments cannot be exposed by the study of the physical world (science) is to deny that Aquinas really meant what he said. Which is it? Was he simply performing a mental exercise or did he believe that god is real?

    Regarding the causation argument, again we have the problem of the physical world. In reality, every action has multiple elements, not a single cause.These elements are intertwined and react with each other. For example, what is the “cause” of an apple falling from a tree? For each individual apple that falls from a tree you cannot describe only one “causation”. There are many elements involved. For example. The temperature, age of the apple, type of tree, age of the tree, wind velocity, moisture content in the air, presence or absence of outside components like squirrels, earthquakes or even human beings, etc. So, the mere fact that “stuff happens” is hardly proof that god causes stuff to happen.

    So, is Aquinas talking about the real, physical world and postulating a real, physical god? Or is he engaged in interesting mental exercises which are not in any way wrong, but cannot lead one to conclusions about the real world?


    1. Hi, Joseph. I agree with most of what you are saying. The terms that Aquinas employs are unintelligible unless there is a real world relation to the same. So, in a sense we can and should examine claims about change and causality in order to verify or disconfirm them.

      Now, your claim that events have multiple causes is not disputed by any Thomist. There are instrumental causes and efficient causes. Moreover, there are two types of causal series: accidental and essential (called per accidens and per se, respectively. The latter is illustrated in an originator (an efficient cause) through a series of instruments (intermediaries, if you will) to the final cause (or ultimate effect). By definition, an instrument does not contain causal efficacy; it can only transmit it. As Aquinas illustrated, the hand that moves a stick which moves a stone which moves a leaf are all illustrative of instrumental causes. In this series, the leaf’s movement is dependent on the stone, and the stone’s movement is dependent on the stick, etc. But the proximate efficient cause is the person who decided to pick up the stick, etc. The causal efficacy is in the person, not the stick or stone.

      So, even positing multiple causes (in the series above, I did not include gravity, etc.,), Aquinas argues that all per se series must end in an efficient cause. And he, via elaborate argumentation, shows that there is one ultimate efficient cause for all material change, and that is God.


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