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The following is Part VI in a series defending the claims of the Catholic Church. Read Part I here; Part II here; Part III here; Part IV here; and Part V here.

(LifeSiteNews) — In this article we will examine the “third way” by which St. Thomas Aquinas seeks to demonstrate the existence of God.

This demonstration is very closely related to the “second way” and for that reason it is recommended that you read the previous article first, as many of the concepts discussed there are applicable to this argument. 

Summary of concepts examined in the previous article

In the previous article we saw that all the things of which we have sense knowledge are the effect of their formal, material, efficient and final causes.

This means that they are dependent or contingent on the continued operation of their causes. If their causes cease to operate, they cease to exist. Therefore, they are all contingent beings.

A contingent being is something capable of non-existence. It is a “possible” being, but it does not exist of “necessity.” It exists only because it has been caused to exist by a being other than itself.

The opposite of a “possible being” is a “necessary being,” that is, a being whose existence is not contingent on any other being. A “necessary being” would, therefore, be self-existent and uncaused. 

The distinction between contingent beings and necessary beings is central to the “third way.”

Necessity and possibility

As we have seen previously, all St. Thomas’s proofs begin with observation of a reality which all human beings can recognize as true.  

St. Thomas begins his “third way” as follows:

The third way is taken from possibility and necessity. 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.[1]

Our own observations will verify this statement. A man is conceived, grows to maturity, and then dies. An oak tree flourishes for hundreds of years and is then blown down. A nail serves its purpose and then rusts away.

The man, the oak tree and the nail, like all the things of which we have sensory knowledge, are contingent beings. They are not self-existent, if they were they would always have existed and would never cease to be. 

On the contrary, the man, the oak tree and the nail are all dependent on causes external to themselves both for their becoming and for their continued being. They are not a sufficient reason for their own existence, but rather they depend for their existence on something external, and prior, to themselves. 

Therefore, we can conclude that there is no necessity to any of the things of our experience. We are confronted with a universe in which everything we observe is merely possible and nothing is necessary.

And if it is merely possible for each of these things to exist, it is also possible for them not to exist.

As St. Thomas writes:

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.

St. Thomas is arguing that because a contingent being is not a sufficient reason for its own existence, it cannot ultimately be a sufficient reason for the existence of other things.

If everything in existence is incapable of existing prior to being caused by another thing, it would be impossible for any contingent thing to ever have come into existence. This is because there would be nothing in existence to cause it. 

But this is clearly absurd, because we know that the contingent things of our experience actually exist. Therefore, there must be an explanation that accounts for the reality that we experience. 

At this point in the argument, someone may object that while individual contingent beings are merely possible, it may be that the collective of beings, taken together, can account for the generation and corruption of the beings of our experience. 

However, this explanation does not suffice because it fails to explain how a collection of contingent beings can be a sufficient reason for its own existence.  

Philosopher George Hayward Joyce S.J. explains:

If each individual member of a collective whole is such that it cannot account for its own existence, the same must be said of the whole collection, no matter how immense it may be.[2]

And he continues:

It has been aptly said that we might as well say that, although one idiot is not reasonable, a million idiots would suffice to form a reasonable being, as to maintain that an infinite number of contingent beings would constitute necessary being.[3]

And he provides this further instructive example:

Those who contend that while the existence of each substance in a collection is contingent, the collection as a whole may be necessary, are asking us to believe that although each chain in a suspended chain is prevented from falling simply because it is attached to the one above it, yet if only the chain be long enough, it will, taken as a whole, need no support, but will hang loose in the air suspended from nothing.[4]

Just as a chain cannot be its own support – no matter how long – neither can a collection of contingent beings ever be a sufficient reason for its own existence. 

That is, if everything in the universe is merely contingent and dependent for its existence on something external to itself, then none of these things can account for their own existence, or that of anything else, prior to having been caused. 

Therefore, we must conclude with St. Thomas that:

Not all beings are merely possible, but there must exist something the existence of which is necessary.

This necessary being is uncaused and self-existent, as St. Thomas explains:

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.[5]

Therefore, the conclusion is inescapable:

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.

Conclusions drawn from the first three ways

We have now examined the first three of St. Thomas’s “five ways” of proving the existence of God.

You will have noticed that they are all very similar to each other, in some respects they can be considered the same proof considered from different aspects. They each have a different starting pointing, but they follow the same general line of argument. 

The “first way” begins from the undeniable observation that the things of our experience change. 

The “second way” begins from the undeniable observation that the things of our experience are caused by something other than themselves. 

The “third way” begins from the undeniable observation that the things of our experience are merely “possible” and not “necessary.”

From these observations, it can be demonstrated that there must be an “Unmoved Mover,” an “Uncaused Cause,” and a “Necessary Being.” We have seen that other divine attributes can be derived from these attributes. Such a being must be eternal, infinite, perfect, personal, and the creator and sustainer of all things that exist. In future articles we will demonstrate all of this clearly.

However, in these conclusions, we already have the foundations of the true religion, as Désiré-Joseph Cardinal Mercier writes:

We are now able to draw from the proofs of the existence of God which we have established, an important moral consequence; namely, the foundation of religion. If we are indebted to a first Being for our essential perfections, our activity and our existence, there is a moral obligation on us of worshipping that Being. The act of worship whereby we confess that God alone is He who is and that our being and perfections are essentially dependent upon Him is that of adoration. And adoration, which embraces the internal avowal of the human soul that it is subordinate to Him and the external manifestation of this avowal, is the fundamental act of natural religion.[6]

Thus, the human being is moved from the recognition of God’s existence, to the worship of God, and, ultimately, as we shall see, to the acceptance that this God has revealed himself to mankind in Jesus Christ. 


1 All quotations from St Thomas Aquinas are from Summa Theologica, I. q.2. a.3.
2 George Hayward Joyce S.J., Principles of Natural Theology, (London, 1924), p81
3 Joyce, Principles, p81-2.
4 Joyce, Principles,  p82.
5 See the previous article for this proof: https://www.lifesitenews.com/opinion/without-god-nothing-else-could-exist/
6 Désiré-Joseph Cardinal Mercier, Manual of Modern Scholastic Philosophy, Vol. II (translated T.L Parker and S. A. Parker) (London, 1917), p46.