You know, I would totally buy this argument if it wasn’t for Aquinas’ last line:
“Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.”
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I wish I had more time to devote to these arguments. Not that I’m some expert, but it would be interesting and help me develop a deeper understanding and learn how best to express it.
I found reading this post by Feser to be useful in better understanding the differences between Paley’s Intelligent Design and the Fifth Way (there are more and better ones, but this one offers more insight into the Fifth Way itself):
edwardfeser.blogspot.com/2009/09/teleology-revisited.html?m=1
By way of clarifying final causality, I will quote this part of it:
The argument isn’t “A tends to cause B; therefore there must be some purpose outside of both A and B the realization of which this causal relationship exists in order to further.” It’s rather “A tends to cause B; therefore, causing B must be inherent or natural to A.”
If that claim sounds obvious and trivial, then terrific: You’re starting to understand Aristotle and Aquinas, because it’s supposed to be obvious and trivial.
Feser and Aquinas both readily admit that while the explanation of final causality is allegedly trivial, moving from there to God is the more difficult step. I’ll try a brief and poor summary. But, just for the sake of argument, try to see this through the lens of there being formal and final causality,because that’s the only way it’ll make sense. It is not compatible with a mechanistic, materialist worldview in which everything is explained in efficient causes. I’m not saying you have to agree with me, but it won’t even begin to click if you don’t understand final causality.
An acorn has the oak as its final cause. But there is no oak tree in the acorn itself. However, if the acorn is directed to this end even prior to the oak tree, then the oak tree must exist somewhere (take a few minutes on that one before immediately replying. I’m not saying you’ll end up agreeing, but it’s a point that does require a little stewing). As stated, it doesn’t exist in the acorn. And Aquinas rejects the idea of some realm of Platonic forms. The third option, then, is that the end exists in the divine intellect. This is true for all ends in which the object is not its own end (important point, for this requires a necessary being which is its own final cause (the proof), and that being is what we’d call God). They are directed towards an end, so the end must exist in some way in order for any type of ordering
towards it to make sense. If an object is pointed towards something, you can’t then say it isn’t pointed towards anything.
And it should be clear that God isn’t assumed in this argument in a circular way. We start with only the existence of final causality. But once we work out that any type of ordering towards certain outcomes can’t exist in any contingent being, then, by necessity, for objects to be directed towards something, there must be a being with an intellect for it to reside in, and if that being were not its own final cause but directed towards something else, then that being’s end must exist in an intellect, so such logic continues until we reach a necessary being who is its own end. And as Aquinas says, “and this being we call God.”
This is my first time attempting to explain this. There are further arguments as to why it must be one being and some other criteria, but I hope that can make sense as some type of starting point about what Aquinas meant, even if you don’t buy it.
And by end, I don’t mean to imply everything has some divine purpose. It just meams it’s ordered to produce a certain effect under certain conditions. An acorn grows into an oak, not a bunny. Am electron orbits a proton, not poofs into a bouquet of flowers. A brick pushes against/through glass, not melt upon contact (barring secondary causes). I think teleology develops to higher tiers in systems, especially biology. DNA is highly teleological. It’s a string of molecules that points towards certain a certain creature with certain traits and certain processes, and is often likened to “blue prints” (though it’s more nuanced, I know). But even inorganic materials are directed towards certain outcomes/ends, etc… as explained.