The Third Way: Aquinas’ Argument from Contingency

*Note: Aquinas’s third way is probably the most difficult to understand of the five ways, and there is disagreement amongst Thomists about how exactly to interpret it. The following is my own understanding of what Saint Thomas is trying to do in the argument.

The Argument

  1. We find in nature contingent beings, i.e. things which are possible either to be or not be
    1. We find beings which are generated and corrupted
    2. Whatever is generated and corrupted is possible either to be or not be
  2. Contingent beings do not always exist
    1. Whatever is contingent is generated and corrupted
    2. Whatever is generated and corrupted does not always exist
  3. If everything were contingent, then at some point nothing would have existed
  4. If at some point nothing were in existence, nothing would be in existence now
  5. Things are in existence now
  6. Therefore, not all beings are merely contingent; at least some necessary being exists
  7. Every necessary being has its necessity caused by something, or not
  8. It is impossible to go on to infinity in necessary things which have their necessity caused by another
  9. Therefore there exists some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity.

The Defense

The third way is the most complex of all the arguments we’ve looked at so far. Some important things to note up front: the third way is radically different from other arguments from contingency. Most other contingency arguments usually contend that whatever is contingent has an explanation, the universe is contingent, and therefore the universe has some explanation. The third way operates using a vastly different understanding of “contingency.”

Secondly, at first glance the argument admittedly looks rather weak, if not downright fallacious. I hope to show in the following defense why that is not the case.

For Aquinas, a “contingent” being is a being which is possible either to exist or not exist. There is nothing about its own nature which necessitates either that it exists or not exists. How do we know that there such things? Because we perceive in nature that there are things which are “generated” and “corrupted”.

In a broad sense, generation and corruption just mean coming into existence and going out of existence. An animal is born and becomes a living creature; an animal dies and becomes an inanimate corpse. But more specifically, generation and corruption have to do with substantial changes in matter and form. For Aquinas, physical things are composite of matter (e.g., the bronze of which a statue is made) and form (what separates an actual sculpted statue from just a large piece of unshaped bronze). When something is generated, the actual underlying “matter” doesn’t come into existence, but a new substantial “form” does. Physical things are generated from prior things (a zygote from a sperm and egg, etc.); and when this occurs, the underlying material substratum undergoes a change of substantial forms, from one form to the other. Similarly, when something is corrupted, the actual matter does not pass out of existence, but an old form is replaced by a new one.

Why does generation and corruption show that something is a contingent being, i.e. that it is possible either for it to exist or not exist? Because if something is generated and corrupted, that means there is nothing about its nature which necessitates that it exist or stay in existence.

The next premise might seem to be unwarranted: “Contingent beings do not always exist.” Since most people understand a contingent being to just be something which has a reason for its existence external to itself, there’s nothing to necessitate that such a being could not always exist, if its reason for existing always causes it to exist. But for Aquinas, remember, a contingent being is specifically a being which is generated and corrupted, and it makes no sense to say that something which is generated and corrupted could always exist. If something begins to exist and at some point ceases to exist, then of course it does not always exist.

The third premise is what makes the argument as a whole so controversial. Aquinas is very often accused here of making what’s known as the “quantifier shift fallacy.” The quantifier shift fallacy is when one makes a jump from something like “Everyone alive was born at some time” to “Therefore, there was some time at which everyone alive was born.” In other words, it is fallacious to argue from the fact that everyone had a time at which they were born, to the conclusion that everyone must have been born at the same time.

Opponents of the third way often suggest Saint Thomas is making exactly this mistake when he jumps from “that which is possible to be at some time is not” to “if everything is possible not to be, then at one time there could have been nothing in existence.” But, in fact, when one understands what Saint Thomas is really saying, it becomes clear that no such fallacy is being made.

Remember, “contingent” beings as we’re calling them here are things which are generated and corrupted, i.e. things which undergo a change of substantial form in an underlying material substratum. But the underlying material substratum itself — or “matter” simply — cannot possibly be such a contingent being. Matter is precisely the thing which enables substantial change — and hence generation and corruption — to take place, by persisting through the process. Matter must exist to make generation and corruption possible; so matter itself cannot be something which undergoes generation and corruption.

But now suppose that everything were contingent beings, and there were no underlying material substratum. Since a contingent being is something which comes into existence and does not always exist, if there is no matter, then a contingent being must have come into existence from nothing (i.e., with no material cause).

So if everything is a contingent being, then everything must have come into existence. And since there is no underlying matter, everything would have had to come into existence from nothing. But something cannot come into existence from nothing. If some Object A, in order to come into existence, must come into existence from nothing, then Object A will not ever come into existence, and hence will never exist. If every object, in order to come into existence, must come into existence from nothing, then every object will not ever come into existence, and hence nothing would ever exist.

And, as is obvious, if nothing ever existed, nothing would be in existence now. But things are in existence now. So it cannot be the case that there was ever a point at which nothing existed, and thus it cannot be the case that everything is a contingent being. There must be something, at least one thing, which is not contingent, i.e. which is not generated and corrupted. There must be at least one “necessary” being.

Here we see another crucial difference between the third way and other contingency arguments. Because of the understanding of contingency and necessity most other arguments use, the argument is usually considered finished as soon as it reaches a necessary being. Not so with the third way. As should be clear from what we’ve already said, the third way considers mere “matter” to be a necessary being, in the relevant sense. To be necessary just means to not tend to undergo generation and corruption. Matter does not tend to undergo generation and corruption, but rather is a necessary prerequisite for any objects to undergo generation and corruption.

So there is at least some necessary thing. The next stop is logically self-evident: a necessary thing has its necessary either in itself, or from another. In other words, a necessary thing either has its necessity intrinsically, or derivatively. If the latter, a thing is necessary because it depends upon the necessity of something else. But there cannot be an infinite regress of a series of such derivative dependence (for a defense of the impossibility of infinite regresses, see our posts on the first two ways.)

So there must be at least some being which is necessary and whose necessity is not derivative. There must be a being that is necessary in itself, and which causes the necessity in other things.


Supposing there is a necessary being that has its necessity in itself, not derived from another, and which causes the necessity in other things. Why think that this first necessary being is God? 

First, a being which has its necessity of itself must be eternal. A being which has its necessity from another could in theory begin to exist. Since it derives its necessity from something else, that something else could only begin to cause its necessity at some specific point in time. But a being which is necessary in itself will always exist, since it is intrinsic to its own nature to exist, and is not dependent upon anything other than itself for existence.

A being which has its necessity of itself is a being whose essence and existence must be identical. For Aquinas, essence is what something is (its nature), and existence is that something is, i.e. whether it is real or not. For most things, these are distinct. The essence of a tree cannot include existence, because trees don’t always exist. In fact, for anything that is contingent, or anything that is necessary but derives its necessity from another, essence and existence will be distinct. But for a being whose necessity is from itself, essence and existence will be identical. It will be a being whose essence just is existence, i.e. Existence Itself, or Pure Existence.

But a being that is pure existence is also pure act, for actuality and existence are equivalent. A being that is pure existence could not possible have any potentials. And thus we must conclude that the being which is necessary in itself must be identical to the being of pure act arrived at in the previous two ways. And a being of pure act must necessarily have the qualities of immutability, immateriality, incorporeality, eternality, and timelessness. And these are the classical attributes of God. (For a defense of pure act having these qualities, see the previous two posts, here and here).

The argument establishes that matter is a necessary being. But why couldn’t matter be that which has its necessity of itself? How do we know that matter must derive its necessity from something else?

We know that a being which has its necessity of itself must be pure existence and hence pure act; and we also know that a being of pure act must have certain attributes. Those attributes include immateriality and immutability. Obviously, matter cannot be immaterial. Nor can matter be immutable, since matter must be that which underlies all substantial change. So we know matter cannot have its necessity of itself.

If matter is a necessary being, does that mean matter has always existed, and never began to exist? Wouldn’t that conflict with the Christian doctrine of the universe having a beginning?

That matter is necessary means that matter cannot be generated or corruption, not that it cannot have an absolute beginning. Remember that technically generation means undergoing a change in substantial form, which is impossible for matter. But Saint Thomas distinguishes generation from “creation”. Generation requires an underlying substratum, creation does not. Generation requires a material and efficient cause, creation requires only an efficient cause.

So the third way takes us to a being that is necessary, that has its necessity in itself, that is existence itself, pure act, immutable, immaterial, incorporeal, eternall, and timeless.



Quotations from the Summa Theologiae are taken from this online source:

My interpretation of the third way is influenced by this post:

Other sources:

Aquinas. On the Principles of Nature.

C. F. J. Martin. Thomas Aquinas: God and Explanations. Edinburgh University Press, June 30, 1997.

Feser, Edward. Aquinas. London: Oneworld Publications, 2009.




6 thoughts on “The Third Way: Aquinas’ Argument from Contingency

  1. Harrison, you write:

    We know that a being which has its necessity of itself must be pure existence and hence pure act; and we also know that a being of pure act must have certain attributes. Those attributes include immateriality and immutability

    Your whole column is quite good, but you should at least link to an explanation why a being of Pure Act must be immaterial. Anybody stumbling across your site would be very frustrated without links to obvious questions.

    I know they can ask in the comments section, but why not at least point them in the right direction?


  2. Anyway, for the interested reader, this is what Harrison wrote elsewhere:

    But is this true for all material objects? It seems somewhat certain that it is, for being material just is being changeable. Consider a single hydrogen atom. If I were to add a proton, it would become a helium atom. Or consider the proton itself; it has the potential to be in any of the elements. An electron in a hydrogen atom has the potential to be in a higher or lower energy state. The subatomic particles have the potency to be rearranged in a plethora of different ways. The same is true for whatever the most basic particles of physical reality turn out to be; physical particles are malleable and mutable, they have the potential to be a part of an almost endless amount of material structures. Some particles make up planets, some stars, some comets, some black holes, some tables, some elephants, some humans. This is all to say that whatever the most fundamental structures of reality turn out to be, they will be absolutely full of potencies. So “matter” and “energy” are about as far away from Pure Act as you can get. The being of Pure Act, therefore, being necessarily immutable, cannot be material, for to be material just is to be mutable, and full of potency.

    Liked by 1 person

  3. Harrison, will you please explain to me the difference between form/matter and nature/supposit? I’m reading Dolezal’s God Without Parts (excellent read), but his definitions of said terms appear so similar, I cannot detect the difference.


    1. I haven’t read that particular book, but I understand where the confusion between the two comes from. Both are related to the act/potency distinction. Form is act, matter is potency that receives the act of form. Now, substantial form (say, of a man) and prime matter together constitute the individual *substance* of a thing. In abstract, the substance of a thing includes both form and matter. This corresponds to it nature. For instance, the nature of a man is a rational animal, and this definition includes its form and material aspect (since an animal is by definition a physical, fleshy thing). All humans share this same nature. Supposit is what it is for this general, abstract nature to be instantiated in *this particular* individual human. You and I both have matter, but we don’t have the *same* matter. The supposit is what separates the instantiation of human nature in one individual vs another individual.

      Liked by 1 person

      1. Thanks much! That’s much clearer than Dolezal’s explanation. Overall, Dolezal does a good job walking a beginner through all the terminology, but he occasionally glides over explaining himself; hence, my confusion.

        Liked by 1 person

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