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

(LifeSiteNews) — In the previous installment of this series, we asked whether the existence of God was self-evident.

We concluded that the existence of God is not self-evident, in the sense of being immediately and intuitively known by human beings.

However, truths which are not self-evident to us can still be known with certainty. This can be achieved either by demonstration or by the use of evidence.

What is a demonstration?

In a demonstration, a truth is proven by logical argument in such a manner that the person who understands each stage of the reasoning process is compelled to recognize that the conclusion is true.

Demonstrations can be used in sciences such as mathematics and philosophy, where a correct reasoning process, applied to truths already known, can lead the mind to conclusions which are seen to be certain.

There are some truths which cannot be logically demonstrated in the above manner, but which can nonetheless be shown to be true by use of evidence. For example, the use of historical documents can prove that certain events happened, for example that Napoleon invaded Russia in 1812, or that Berlin was captured in May 1945. Likewise, while a prosecutor cannot provide a logical proof that a certain person committed a crime, he can set out evidence sufficient to convince a jury of his guilt beyond reasonable doubt.

A truth which is self-evident cannot be demonstrated. This is because it is immediately and intuitively known. In fact, demonstrations of any truth are based on the use of first principles which are self-evident, and without which our reasoning process could not advance to further reliable conclusions.

Therefore, it is impossible for the existence of God to be self-evident to us and also capable of demonstration.

As the existence of God is not self-evident, we can ask whether it is a truth capable of demonstration.

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The opponents of the natural knowability of God

There are those who do not believe that the existence of God can be demonstrated by natural reason.

Agnostics hold that, whether God exists or not, our reason is not capable of reaching certain conclusions about His existence or His nature. There have been many schools of agnosticism. That of Immanuel Kant (1724-1804) is one of the most influential. Kant regarded human reason as being incapable of reaching certain conclusions about that which is beyond the range of the senses.

The heresy of Modernism is a form of agnosticism, which owes much to the approach of Kant. In his encyclical letter Pascendi Domenici Gregis, “On the Doctrine of the Modernists,” Pope St. Pius X noted that:

Modernists place the foundation of religious philosophy in that doctrine which is usually called Agnosticism. According to this teaching human reason is confined entirely within the field of phenomena, that is to say, to things that are perceptible to the senses, and in the manner in which they are perceptible; it has no right and no power to transgress these limits. Hence it is incapable of lifting itself up to God, and of recognizing His existence, even by means of visible things.[1]

As a result of his agnosticism, the Modernist does not believe that man can assent to a supernatural revelation made by God. Instead, he believes that Catholic doctrine can only be the symbolic representation of internal human experiences.

Fideism also denies the ability of natural human reason to attain certain knowledge about God. There have been many different forms of fideism, but generally speaking they emphasize the act of faith, while denying the possibly of certitude. One form of fideism is the nineteenth century heresy known as traditionalism. The proponents of this idea believed that the existence of God was known only by accepting, by faith, a revelation which had been handed down through the generations. They denied that unaided human reason could come to the certain knowledge of God.

The knowledge provided by our senses is certain

Against the above errors stands the thesis of the natural knowability of God, which was expressed by St. Paul as follows:

For the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity. (Rm 1:20)

This is the position held and defended by Catholic philosophers and theologians for twenty centuries. It has also been defended by many outside the Church, pre-eminent among whom is the ancient philosopher Aristotle.

In the introduction to this series, I explained that the first installments would treat of that branch of philosophy known as Natural Theology. But we must first briefly summarize a few of the conclusions reached by the branch of philosophy called Logic. Logic is the science which studies both the processes of human thinking and the truth and certitude of our conclusions.

The foundation of all human knowledge is the sense knowledge that we acquire through our five external senses of touch, smell, taste, hearing and sight.

These senses, when they are free from physical disorder and applied to their proper object, yield certain knowledge. For example, a healthy eye brings us certain knowledge about color, a healthy ear brings us certain knowledge of sounds, and so on.

In practice, no-one doubts that this is the case. The greatest skeptic sits down on the chair which his eyes perceive to be present, without expecting to collapse onto the floor instead. He then drinks the coffee which he has brewed without any expectation that what looked and smelled like coffee, will turn out to be tea. He looks out of the window and knows that it is day rather than night, and so on. The reliability of the senses, when not impaired by injury of the sense organ, is the basis of human thought and human action.

Sense knowledge leads to intellectual knowledge

Our senses bring us knowledge of individual objects. We see the drawing of this particular circle. We see, through our sense of sight, this particular tree; we can also touch its bark, smell and taste its fruit, and hear the wind rustling through its leaves.

As human beings, with intellectual souls, we are able to abstract from the sense data that we perceive and form ideas. We can know what the essence of “circle” or a “tree” is, in a way which abstracts from any particular circle or tree. We now know what is true of all possible circles, and of all possible trees. The certainty of our sense knowledge permits a grasp of these universals which is also certain. There are simple intuitive ideas, which we form immediately upon sensation, and there are other more complex ideas which we derive from those which are simpler.

As we form ideas, we begin to compare them with each other, and see where they agree, and where they disagree. For example, the ideas “man” and “animal” agree in so far as men and animals are sentient living bodies, but they disagree in that men are rational while other animals are not. This operation of the mind is called judgment.

Judgments are expressed as propositions. A proposition is a formula consisting of a subject and a predicate. The example that we saw in the last article was “Man is a rational animal,” whereby “man” is the subject and “is a rational animal” is being asserted of it.

Judgements expressed as propositions form the basis for the process of reasoning by which we infer, or draw out, other truths implicit in two propositions. The fundamental expression of this human reasoning process is the syllogism, whereby a certain conclusion is drawn from two premises. For example:

Major premise: Reykjavik is in Iceland

Minor premise: John lives in Reykjavik

Conclusion: John lives in Iceland

If the premises are true, the conclusion must also be true. The conclusion can then be included in a new syllogism with a new premise. If both of these premises are true, a further conclusion will be reached, and our knowledge will have advanced again. All human reasoning is by nature syllogistic, though we may not consciously be aware of this.

It is important to be aware, of course, that a conclusion is only as sound as its premises. If a premise is false, then the conclusion may also (but not necessarily) be false. Thus, human thinking is often erroneous.

For example:

Major Premise: All living things are rational

Minor Premise: A tree is a living thing

Conclusion: A tree is rational

This conclusion is false, because the first premise is false.

However, the important thing for us to understand here is that it’s possible to:

  1. Obtain certain sense knowledge from our senses
  2. Form ideas from sense knowledge that represent the essences of real things
  3. Make true judgments by comparing two ideas
  4. Correctly infer further truths from these judgments by means of our natural reasoning process.

The conclusions arrived at by this process will be certain, in the absence of defects at any of the above stages.

Knowing a cause from its effect

It is not possible for our external senses to gain sense knowledge that enables us to form an idea of the divine essence.

However, it is not necessary to sense a thing directly in order to know of its existence and to know something of its nature.

We can gain certain knowledge about one thing by the knowledge that we possess about another thing.  We can:

  1. know an effect from its cause and
  2. know a cause from its effect.

Imagine that a gun is to be fired at a target. From your knowledge of the gun, you could know a great deal about the effect it will have on the target before the shot was fired. The conclusions that you formed about the nature of the effect, would be determined by the nature of the cause.

This kind of argument is called a priori, it argues from what has come before, to demonstrate what the effect will be.

On the other hand, imagine examining a target after it had been shot. By examining the effect, you will be able to extrapolate back to the existence of the gun. The existence of the gun is certainly known, even though you did not see the shot being fired. And the more information about the effect that you gather, the more you will know about the type of gun which was used.

This kind of argument is called a posteriori. The existence and nature of the cause is demonstrated by its effects.

Application to the existence of God

We saw in the previous article that the word God signifies the supreme being who is the creator and ruler of the universe.

If we are attempting to demonstrate the existence of a God who is the cause of all things, we cannot attempt a proof by a priori argument. God, by definition, is not the effect of another cause, and therefore we cannot argue from such a cause that He exists.

On the other hand, we are proposing that God is the cause of an effect – namely material creation. Therefore, we can study the nature of that effect, and, from it, attempt to determine the nature of the cause. That is, we can attempt a demonstration a posteriori.

This demonstration is what we will attempt in the next installment of this series.

The natural knowability of God as a dogma of the faith

So far in this article we have been approaching the question of the natural knowability of God from the perspective of natural reason.

That is because in this series we are exploring how the truth of the Catholic faith can be known with certainty by everyone, as the result of a reasoning process which begins with the certain knowledge provided by our senses and ends with an act of supernatural faith.

However, to conclude this article I wish to briefly consider the question from the perspective of one who has already made his submission to the infallible teaching authority of the Catholic Church.

A person who approaches the question from this perspective knows with certainty that it is possible to demonstrate the existence of God by use of natural reason, because this proposition is proposed to us by the Church.

The Vatican Council (1870) taught the following:

Holy Mother Church holds and teaches that God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason: ever since the creation of the world, his invisible nature has been clearly perceived in the things that have been made.

In the same session of the Council, the contrary error was condemned:

If anybody says that the one true God, our Creator and Lord cannot be known with certainty in the light of human reason by those things which have been made, let him be anathema.[2]

Consequently, to deny that natural reason can know with certainty from created things that God exists is heretical.

In 1910, in the Oath against Modernism, Pope St. Pius X mandated the profession of this truth:

I profess that God, the origin and end of all things, can be known with certainty by the natural light of reason from the created world, that is, from the visible works of creation, as a cause from its effects, and that, therefore, his existence can also be demonstrated.

In this passage St. Pius X specifically requires acceptance of the position that the existence of God can be known by a demonstration “from the visible works of creation, as a cause from its effects,” that is, a posteriori.[3]

In the next article we will proceed to making such a demonstration.


1 Pope St Pius X, Encyclical Letter Pascendi Dominici Gregis, “On the Doctrines of the Modernists”, No. 6.
2 First Vatican Council, “Dogmatic Constitution on the Catholic Faith” (Session 3, 24 April 1870).
3 This excludes the position that God’s existence is self-evident because that which is self-evident is not capable of demonstration.