St. Thomas’ Five Ways all have the same structure: P exists, P is being caused by Q, so Q exits. As the Five Ways advance, however, the term I here call “P” goes from being more known to us to being more known in itself, and as a consequence it goes from being something more extrinsic to the things we know to being something more intimately within them.
The First Way, which St. Thomas says is most manifest to us, is taken from motion- which is preeminently knowable to us. Motion is a per se sensible- in fact it is one of the few sensible things that all sense organs are aware of. Also, experience shows that we know motion in an exact way even before we know the thing in an exact way. At the same time, motion is the least actual of actual things, for it is the first immediate level above what is not actual at all.
All of the proofs after the first are based on things that are not sensible per se, but which are properly known by intellect. Because each rests on a basis known properly by intellect, all of the proofs penetrate into what a thing is, as opposed to resting in the sensible appearances of things. And yet, because they are not known to sense as such, all of them fall away from what is known to us first. We must add the important caution, however, that to deny that something is sensed as such does not mean that it fails to be sensed in any way.
The Second Way is based on efficient causes. When we speak of causes, the efficient cause has a certain primacy, for whenever people speak of a cause unqualifiedly they tend to be speaking about an efficient cause, because cause means first “being responsible for something”, and the first thing we realize as responsible is the one who did the act, or the agent. But even though when taking about causes we tend to know the efficient first, this cause is first understood as being extrinsic to the thing we are studying, like the painter is extrinsic to his portrait.
The Third Way, which I based on contingency and corruptibility in things, overcomes this exteriority of efficient cause and gives us the first kind of cause that we recognize as within the thing itself, because contingency and corruptibility are the result of matter, which is the most knowable interior principle of things.
The Fourth Way, because is based on degrees of perfection, is based on form, which is more perfectly an interior principle of things. It is a more perfect cause because it is prior in causality to matter, and it is in fact that in virtue of which this matter is appropriated to something or rejected by it. Form, however, is known by us only with difficulty, and as a rule those who study natural things do not ascend to the level of being able to recognize the substantiality of form.
The Fifth way is based on the determination of things to something, or what is called “final causality”. This last kind of cause is in fact the cause of all causes, and it most fulfills the thing we name when we speak of a cause. Because of this, this last kind of cause is most within the things we know, and it most constitutes them in being- which I why a definition taken in the order of final cause can govern and explain all the other causes of the thing. Because of its preeminent causality and knowability, however, it is least knowable to us.
In the ascent of the Five Ways, each of the proofs is based on something that more and more manifests the divinity that is responsible for it. Motion, for example, barely manifests divinity at all, except perhaps insofar as it is a sign of life and vitality. It does, however, have a preeminent knowability and is the ground for all our knowledge of causality. Efficient causality prompts us to seek for a first cause, for we recognize a given finite order. Material causality first shows us something eternal, for matter understood as a principle of change neither comes to be nor passes away. Form, as Aristotle says, is “a divine thing”, so much so that Plato would argue that they could only exist in the heavens as both eternal and perfectly intelligible. The final causality within a thing, however, preeminently manifests the existence of God who is responsible or such causality, so much so that almost all recognize the truth of the consequence “if nature acts for the sake of something, God exists”. All of this is a consequence of the fact that the more one penetrates into the reality of natural things, the more they are removed from what they understand by sense, but the more they attain to what manifests the divine life.
Just Thomism 