St. Thomas Aquinas:

On The Existence of God

Is the Existence of God Self-Evident?

Objection 1. It seems so, for those things are said to be "self-evident to us" the knowledge of which exists naturally in us, as is clear in the case of first principles. But as [Paul] says in the beginning of his book, "the knowledge of God's existence is naturally implanted in all." Therefore the existence of God is self-evident.

Objection 2. Moreover, those things are said to be self-evident which are known as soon as the terms are grasped, as [Aristotle] says in Book I of the Posterior Analytics. Thus, when the nature of a Whole and a Part is known, it is at once recognized that every whole is greater than its part. But as soon as the meaning of the word "God" is understood, it is at once seen that God exists. For by this term is signified a reality than which nothing greater can be signified.

Objection 3. Then, too, the existence of Truth is self-evident. For he who denies the existence of Truth, concedes its existence. For if truth doesn't exist, then the statement, "Truth does not exist" is true. And if there is anything true, there must be Truth. But God is truth itself: "I am the way, the truth, and the life" (John 14:6.) Therefore, God's existence is self-evident.

Reply: It may be that one who hears this name "God" does not understand it to signify something than which nothing greater can be thought. For some have believed God to be a body. But even if it be supposed that everyone understands that by this name "God" is signified something than which nothing greater can be thought, it does not follow that he understands that what the name signifies exists actually, but only that it exists mentally. And it can't be argued that it actually exists, unless it is admitted that there actually exists something than which nothing greater can be thought. This is what is not admitted by those who hold that God does not exist.

Can it be demonstrated that God Exists?

Objection 1. It seems not, for it is an article of faith that God exists. But what is of faith cannot be demonstrated, because a demonstration produces scientific knowledge, whereas faith is the unseen, as is clear from Hebrews 11:1. So it cannot be demonstrated that God exists.

Objection 2. Moreover, we cannot know what God is, but only what He is not, as [Paul] says. Therefore, we cannot demonstrate that God exists.

Objection 3. Then too, if the existence of God were demonstrated, this could only be from His effects. But His effects are not proportioned to Him because He is infinite, and His effects are finite, and between the infinite and the finite there is no proportion. So, since a cause cannot be demonstrated through an effect not proportioned to it, it seems that the existence of God cannot be demonstrated.

On the contrary: Paul says: "The invisible things of God are clearly seen, being understood by the things that are made." (Romans 1:20). But this would not be unless the existence of God could be demonstrated through the things that are made. For the first thing that must be known of anything is whether it exists.

I answer: Demonstration is two-fold. One type is through the cause, and the other is through the effect. When an effect is more obvious to us, then we proceed from that effect to knowledge of the cause. And from every effect, the existence of its cause can be demonstrated, so long as its effects are known to us. Since every effect depends on its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, being not self-evident to us, can be demonstrated through effects which are known to us.

Does God Exist?

Objection 1. It seems not, because if one of two contraries be infinite, the other would be wholly eliminated. But the very word "God" implies infinite goodness. So if God existed, there would not be evil. But evil is found in the world. Therefore God does not exist.

Objection 2. Moreover, that which can be brought about through fewer causes has not been produced by many. But it seems that everything we see in the world can be brought about through other causes, if God didn't exist. For all natural things can be traced to one cause, which is nature. And all voluntary things can be traced to one cause, which is human reason or will. So there is no need to suppose that God exists.

I answer: The existence of God can be proved in five ways.

The First and most manifest way is taken from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now, whatever is being moved is being moved by another. If that by which it is being moved be itself moved, then this also must be moved 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. For secondary movers move only because they are themselves moved by the primary one. That is why it is necessary to conclude to a First Mover - which is moved by nothing else whatever. And this all understand as being "God."

The Second way is from the nature of the efficient cause. Among sensible things in our world we find an order of efficient causes. There is no case of a thing being the cause of itself, nor is such a thing possible, for then it would exist prior to itself, which is impossible. Therefore it is necessary to admit a first efficient cause, which we call "God."

The Third way is taken from the possible and necessary. We find some things which are possible of being or not-being, since they are found to be generated, or corrupted, and consequently possible of being or not-being. But it is impossible for all such things always to exist, for that which can not-be at some time is not. So if everything can not-be, then at one time there was nothing. But if this were true, even now there would be nothing, because that which is not does not being to Be, except by something which is. So, if at one time nothing existed, it would have been impossible for anything to have begun to exist. Therefore, there must be something whose existence is necessary. It is therefore necessary to admit the existence of a being having its necessity through itself, and not receiving it through another, but rather causing in others their necessity. This we call "God."

The Fourth way is taken from the grades of perfection found in things. Among beings there are some more and some less good, true, noble, etc. But "more" and "less" are predicated [assume] a maximum, as a thing which is "hotter" is only so because it is approaching "hottest." There is, then, something which is Truest and Best and Most Noble, etc. In other words, something which is a maximum, and this we call "God."

The Fifth way is taken from the governance of things. For we see that certain bodies devoid of natural knowledge act toward an end. This is evident from their acting always in the same way, in order to obtain the best result. Hence it is clear that they achieve their end, not by luck, but by purpose. Now whatever lacks knowledge does not tend toward an end unless it is directed by some being endowed with knowledge and intelligence, just as the arrow is directed by the archer. It follows that some intelligent being exists by whom all natural things are directed to their end. This being we call "God."