I will go through this article slowly; very slowly. Just a paragraph or two at a time. It is long, and sometimes technical. The article can be found here.
The Fifth Way, as often remarked, depends on final causality, explanation in terms of an end, of what is the point of things. The aim is to show that the world, like its parts, requires an explanation in terms of what it’s for, and what the world is for is set by God. It is a mistake to say that this argument is based on the world’s being for a purpose: it is rather based on the world’s having a point, which is at first sight quite another thing. The circulation of my blood has a point, but it does not have a purpose: nor do even I have a purpose in circulating my blood, since circulating my blood is not something in the strict sense which I do. I can’t help it. It is true that in order to reach God Aquinas has to argue that the point of the world has to be a purpose set for it by God, but that is a further step in the argument.So explanation in terms of an "end" is different from explanation in terms of a "purpose." Martin's circulatory system has an explanation in terms of an "end", of "what it's for": to deliver oxygen throughout his body. But it does not have a purpose: it might have been created directly by God, or by blind evolutionary forces. Whether it is a part of something larger that has a purpose or not, considered as a part of the system of his body, Martin's circulatory system only has an end, not a purpose.
This is the briefest of all the Five Ways, and its structure is apparently clear.This is his formalization of the Fifth Way.
1. We see that there are things that have no knowledge, like physical bodies, but which act for the sake of an end.
2. But things which have no knowledge do not have a tendency to an end unless they are directed by something that does have knowledge and understanding.
3. Therefore there is some being with understanding which directs all things to their end, and this, we say, is God.
But this simplicity is deceptive, and the history of natural theology and apologetics has made it yet more difficult for us to read this argument as we should. Most people would want nowadays to object to the first premiss, and to deny that we see that physical bodies act for the sake of an end. But they would grant that if we saw physical bodies acting for the sake of an end, we should conclude immediately to the existence of God. Aquinas does not agree, I think. He thinks that it is almost unquestionable that e.g. physical bodies act for the sake of an end: what is tricky is the step from the unconscious end-directedness which we see all around us to the conscious end-directedness which he needs to assert if he is to prove the existence of God.An important point. One of the key differences between Intelligent Design and the Fifth Way is that in the former, ends and goals are difficult to find but if they are, the existence of a Designer is immediately inferred. However, in Aquinas, it is obvious that physical systems act for the sake of an end; the harder part is getting from that fact to the existence of a Designer. This will become clearer as the article proceeds.
That's enough! I told you this would be short! Snippets. The blog is supposed to be digestible.
The take home points:
- There is a difference between "end" and "purpose"; as Martin will make clear later, something can have an "end" relative to the system of which it is a part, even if that system does not have any overarching purpose.
- In the Fifth Way, the existence of "ends" in the world is easy to find; it is the move from this fact to the existence of God that is difficult.