The Third Way is the most important of the five. In this Way, St. Thomas says that everything in the world depends for its existence on something else. We ourselves depend for our existence on our parents, they on their parents, and so on. But sooner or later we must come to someone who does not depend for his existence on someone else, someone who is himself the reason for his own existence, This someone is God.

In explaining the Third Way, St. Thomas points out the difference between a possible being and a necessary being, that is, between a "might" and a "must". When I think about myself, I can see that I am only a "might". If my father had never met my mother, I would never have come into being. The world I live in is a "might". If the matter from which the solar system was formed had fallen into a different pattern, this planet earth would not exist today. The whole universe is a "might", for it does not contain anywhere in itself the reason for its existence.

So it must depend for its existence on a being who is a "must", in other words, a necessary being. There is no "might" about God. There are no circumstances under which he might never have existed or might cease to exist. He did not come into being, he will not cease to be. He was from all eternity and will be for all eternity. He is the reason for his own existence and the existence of everything else. Nothing caused him, nothing made him. He just is.

The objection is sometimes made that the universe could have always been in existence. But even if it had and if existence had been passed on from one being to another in an endless chain, we would still have to explain where the universe and everything in it got their existence from. If we plug an electric lamp into the mains and it lights up, we know that at the far end of the cable there must be a source of power. It would be no explanation to say that the fire plugged into an endless series of cables, each one drawing power from the one beyond it, because no amount of cables can produce electricity: they can only pass it on.

So, when we watch the things of the world coming into being and passing away again, we know that there must be someone who does not get his being from anything else but is himself the source of all being. If we open our eyes, we can see his signature on everything that exists.

The Third Way is not easy to understand but it is worth the effort because of the profound insight which it gives us into God's nature. He told Moses that his name is Yahweh. The name that best describes him and gives the key to his nature is simply "I AM". Now we begin to see what this means. Other things come to be or cease to be but not God. He is, for ever and always.

As we saw earlier, the people of Israel were not philosophers. When Moses told them the God's name was "He Is", neither he nor they understood very much about what it meant. We understand more but not everything. For this is the very heart of the mystery of God which no one can ever fully grasp.

Three other approaches

Other proofs for the existence of God have been put forward and it is worth saying something about three of them. They are the argument from conscience, the argument from moral order, and the argument from religious experience.

The argument from conscience is based on the knowledge we all have that some of our actions are right and others are wrong. This knowledge is not just theoretical but practical. It comes to us with the force of a law or command which we are under an obligation to obey. If we do not obey, we experience guilt and shame.

We know that we did not invent the moral law and that we have no power to change it. It must therefore have been given to us by a supreme law-giver, who is God. This was the favorite argument of Cardinal Newman who wrote:

The argument from religious experience is based on the fact that some form of religious belief has been found in every country and among every people. Tribes and races completely separated from one another, some of them primitive and others highly civilized, have all worshipped God under one name or another. It is not possible that something so widespread could be without any foundation in reality.

As well as this belief among mankind in general, there are all those individual people who have been given a direct awareness of God which made them certain about his existence and presence. Among these are some of the wisest and noblest people who ever lived, and their word cannot be doubted. St. Thomas Aquinas, for instance, was granted an experience of God shortly before his death. He did not try to describe it except to say, "After what I have seen, everything I have written seems like straw".

Another great man who experienced God directly was the French philosopher and scientist, Blaise Pascal. He jotted down his impressions immediately afterwards and carried the paper around his neck for the rest of his life.

I was in my room. It was night. The light was on. Suddenly it seemed to me that Father, who had now been dead more than a year, was there with me. The sense of his presence was as vivid and as real and as startling as if he had touched my arm or spoken to me. The whole thing passed in a flash, but in the flash, instantly, I was overwhelmed with a sudden insight into the misery of my own soul. And I was pierced deeply with a light that made me realize something of the condition I was in, and I was filled with horror at what was within me, and my soul desired liberation from all this with an intensity and urgency unlike anything I had ever known.

And now I think for the first time in my whole life I really began to pray-praying not with my lips and my intellect and my imagination, but praying out of the very roots of my being, and praying to the God I had never known, to reach down towards me out of his darkness and to help me to get free of the thousand terrible things that held my will in their slavery.

Many of us, perhaps most of us, have had a moment in our lives when we felt the presence of the Being who underlies all the things of the world. It may have come dramatically, a sudden conversation accompanied by tears of joy and sorrow. It may have come quietly and peacefully amid the beauties of nature or of art, filling us with certainty about his presence. We can close our eyes and our hearts to this presence if we want to; but nothing we can do can make it cease to exist. God is.

Summary

We can use our reason to prove the existence of God. We cannot prove his existence with mathematical certainty, since the existence of God is not a mathematical proposition. But from observing the world around us, we can see clearly that it must have had a creator, who is God.

The most famous proofs for the existence of God are the Five Ways of St. Thomas Aquinas. He said that we can see how things in this world depend for their existence on other things. There must be some source of all this existence, otherwise nothing would exist. This source of existence is God, "He who Is"

Other arguments for the existence of God are drawn from the voice of conscience that tells us right and wrong, from the moral order which is upset by man's sinfulness and can only be set right by God, and from the religious belief and experience of races and individuals.