A Helpful Outline for Aquinas' First Four Ways to God in the Summa Theologiae

In St. Thomas' work the Summa Theologiae (I, QII, AIII, I Answer That) he gives a brief summary of five ways in which to come to the conclusion of God's existence. In other places, such as Summa Contra Gentiles (bk1 ch13), he gives more in depth explanations for these proofs. In the Summa Theologiae, though, he keeps it relatively brief. Here's a basic structure to the first four proofs that I have found in reading and talking about these proofs which may be helpful to you as well. 

1) Empirical Observation 

- Motion (Change)
- Efficient Causality
- Generation and Destruction 
- A gradation to the qualities of things

First, in each of them he begins with a empirical observation about the world around us. Thomas' arguments always begin from an effect, which is better known to us, and then reasons to a cause that is less well known to us. These observations are also very fundamental in nature. It is hard to get around the reality that change is occurring in the world, or that cause and effect exists. Thus, his arguments are effective because they begin with realities which are so obvious. To deny them is to deny reality itself altogether. 

2) Principle/Law 

- Nothing can change itself from potency to act as that would be a contradiction.
- Nothing can be the efficient cause of itself or it would be prior to itself.
- There cannot be only possible beings or nothing would exist. 
- Lesser qualities only exist by their dependence on greater versions of those qualities. 

Secondly, Thomas moves to a universal principle of nature regarding those empirical observations. For example, "Things change ... but nothing can change itself." Or, "Causality is all around us, but nothing can be prior to itself as a cause." These principles are at the heart of the argument, but again are so fundamental it is difficult or impossible to get around them without denying a fundamental truth about reality. 

3) Problem/Lack of Explanation

- There cannot be an infinite regress of changes. 
- There cannot be an infinite regress of causes. 
- There cannot be only possible beings and still have an existing reality. 
- There cannot be an infinite regress of greater and greater qualities (Especially the transcendental.)

Third, given the observation and the universal principle of how it works, one is led to seek for an explanation for that observation. Why do things move? Why do things cause other things? Where do things come from when they generate or decay? This inevitable leads to a problem that can only be solved with a very specific logical conclusion. Basically, Thomas is pointing out here that the material universe doesn't hold the explanation for its existence or characteristics within itself. Infinite regresses of changes, causes, or possible beings simply cannot provide an answer the problems encountered. 

4) Solution/Conclusion 

- There must be an unmoved mover. 
- There must be an uncased cause. 
- There must be a being which is necessary through itself. 
- There must be an ultimate version of the transcendental qualities, which is also ultimate in being. 

This finally leads us to the conclusion of each proof. We have to be careful here. Often there is so much baggage associated with the word "God" that many people misinterpret the argument to be assuming God's existence from the start, thus creating a case of special pleading where the restrictions we are placing on material things we are exempting God from. This is to look at the proof backwards. Leave behind the word "God," if that is helpful. Thomas is searching for an adequate cause to explain the effect that we all self-evidently witness. 
Each of Thomas' resolutions to the problems presented in step three can only be an ultimate reality or being. Why? This is because only an ultimate being would not be subject to change, causality, or possibility. Thus, logically there must be some ultimate ground and reality which sustains every other reality. Call it God, call it the prime reality, call it whatever you want, but it must be an ultimate being of pure existence, which as Thomas says, "we usually call God." 

I hope that this basic outline is helpful for you in your reading of the first four ways of Aquinas. 
- Aquinas, Thomas. Summa Theologiae. I, Q2, A3, I Answer that. https://isidore.co/aquinas/english/summa/FP/FP002.html#FPQ2A3THEP1