Aquinas shows ways to show the existence of a God.

See Summa Theologiae 1,2

First Way

This is the longest proof and which Aquinas calls the “most manifest”. Basically it starts off with the idea that, due to our senses, some things are in motion. Now whatever is in motion, must need something in order to put it into motion. Now whatever is in motion is put in motion by another. “For motion is nothing else than the reduction of something from potentiality to actuality, and nothing can be reduced from potentiality to actuality except by something in a state of actuality.” (Summa Theologiae 1,2,1). There cannot be an infinite regress of movers moving things otherwise there would not be a first mover which means that the regress could not exist. “Why can’t the first mover be the universe itself? Because if nothing can move itself, neither can the whole chain of moving things that we call the universe.” (pp 17).

Second Way

The second way is identical to the first way but differs in the fact that it talks about existence and not change.

Suppose I have a book that is on a topic that you like. You ask, “Can you give it to me?”. I respond, “Yes, but I just need to borrow it from my friend”. You ask again, “does your friend have it?” I say no, he needs to borrow it from a library. “Well, does the library have it?” No, they have to find someone who can lend it to them. We can state that if this is infinite, that no one actually has the book, and no one can borrow it.

Now let us say that the book is our existence, I have it, and got it from my parents who in turn got it from their grandparents etc. If no one has it by nature and does not have to get it from someone else, no one would ever get it.  “And the being that can give [us] existence because He has it by his own essential nature is, called God.” (pp 18)

Third Way

This argument begins by stating that everything in the universe is mortal; that is has a beginning and an end. Hence, given the right amount of time, everything would cease to exist. But if the universe had no beginning an is infinite, then there has been enough time for everything possible to become actual. One of these possibilities and the death of everything in the universe. And ergo, if the universe dies, nothing can start because nothing comes from nothing. “You might think of the second law of thermodynamics, the principle of entropy, as an analogy here. All forms of energy tend to flow from higher concentration to lower, to dissipate. Eventually, the universe will be a homogeneous blob in which nothing happens. The only reason this has not happened yet is that there hasn’t been enough time; the universe is only about fourteen billion years old.” (pp 19)

Fourth Way

This is based on the premise that there is a hierarchy of things considered ‘good’ and ‘bad’. If we were to consider something ‘very good’, then there must be by definition something that is the ultimate good. Hence, “The argument could be summarized very simply: one thing is better than

another, and “better” presupposes “best,” and “best” is another word for God.” (pp19)

Fifth Way

This argument initiatives by stating that all things have an end (in other words, goal). Aquinas uses the analogy of an arrow. An arrow is shot with the end of hitting a target. Another example is that puppies have the end of becoming full grown dogs. “The universe is more like a book than like an explosion in a print factory. And if there is a book, there is an author.” (pp 20). For instance, if you were walking along the beach one day, and you saw a perfect letter ‘S’ drawn in the sign, it would be a reasonable assumption that some type of intelligence (i.e human) created it, however there is still the chance that the ocean and wind manufactured the ‘S’. On the other hand, if you saw “SOS” on the sand, it would be much more likely to have been an intelligent design than the utter randomness of the elements.