In his work the Summa Theologica, St. Thomas Aquinas records his five proofs of God’s existence. I have summarized the arguments for a more simplistic understanding for those who have not studied Philosophy. As you read, critically examine which proof is most convincing to you.

The First Proof: Motion

  1. All bodies have the potential to move or are actually in motion.
  2. If something is not in motion (meaning it has the potential to be in motion), it cannot be put into motion unless something in motion acts on it.
    • This means that something cannot be both potentially in motion and actually in motion. It can only be one or the other. 
  3. If nothing can put itself into motion, there was a time when nothing was in motion.
  4. There is motion.
  5. For motion to exist, there must be a “first mover, moved by no other” that is in perpetual motion to put all things into motion.
  6. Therefore, there is a first mover, who we know as God. 

The Second Proof: First Cause

  1. Nothing can cause itself to be in existence. 
    • For something to exist, it must have been caused by something else. 
  2. There is existence. 
  3. If there is existence, there must be a first cause that was always in existence to set everything else into existence. 
  4. Therefore there must be a first cause that is eternal. We know this to be God. 

Third Proof: Possibility and Necessity

  1. The world is full of things that may be and may not be (possible). 
    • Their existence is contingent on something else.  
  2. If everything is contingent, there would have to be a time when no contingent beings were in existence. 
    • This is due to the nature of possible things. 
  3. We have possible things in existence now.
  4. All possible beings coming into existence, through a non-possible being which we call necessary. 
    • A necessary being is a being that must always exist. 
  5. We know this to be God. 

The Fourth Way: Gradation

  1. There is a gradation to be found in things. 
    • We compare things to other things (things are hotter, colder, etc.)
  2. We compare things by saying which one is closer to the ideal of that thing.
    • Bill is better at basketball than Jimmy because Bill is most like the ideal (perfect) basketball player.
  3. If there are things that we call good, there must be something that is the ideal of good.
    • We do call things good. 
  4. The ideal of goodness, and every other perfection; is what we know as God. 

The Fifth Way: Intelligent Design

  1. Everything in nature is ordered. 
    • Ecology, Biology, Chemistry, etc. are concerned with studying this order.
  2. Anything that is ordered either acts out of intelligence, or under the direction of something with intelligence. 
  3. But many ordered things in nature do not have intelligence.
  4. There must be an intelligent being who ordered nature and gave it its purpose. This being we know as God.

In the Middle Ages, these were very convincing arguments. Because the majority of the western world was Catholic, many believed in God and desired to be in relationship with him. As a result, they were able to see the effects of God in their lives. While these proofs are still convincing to many today, there are many who find these unconvincing, because they are not living a life in search of God, or they have no desire to know God. Aquinas’s arguments generally make sense to those who live a Catholic life, but to those who have no desire, they may seem forced. 

The failure of the Catholic Catechist/Priest often is that if the proofs are taught at all, they are trying to convince people through rational thought alone and do not connect the experience of God to the proofs. Many people can be skeptical because of their upbringing, sin, or personal life experiences (or a combination of the three). It is only through the grace of God that these skepticisms can be broken down. It is about this skepticism that Jesus quotes Isaiah, “For this people’s heart has grown dull, and their ears are hard of hearing, and they have shut their eyes; so that they might not look with their eyes, and listen with their ears, and understand with their heart and turn—and I would heal them.”

Even with the experience of God, some arguments may be more convincing than others, and none are dogmatic truths that must be assented to. Yet, the teacher of these proofs has to urge their pupils to have an unbiased, humble heart to find the Truth, For as Jesus tells us, the Truth will set us free.