Even after I began study of Thomist philosophy, the Fifth Way didn’t make sense to me. I had to muse on it, let it stew, and only after some time of that did it really click what Aquinas was saying. And it’s not because Aquinas was being obscure or vague, but it’s because I’m 800 years removed from the philosophical context that he was speaking from. It was something the people of his day would have intuitively understood more easily than myself because they were used to thinking that way. Of all his ways, the Fifth is the most difficult because we are completely unfamiliar with Aristotlean final causes and teleology.
What the Fifth Way is NOT is Paley’s Intelligent Design argument. There’s no “fine tuning” argument or probability argument, and there’s a distinction between substance and artifact as understood by Aquinas that doesn’t fit Paley’s mechanistic world view, in which the Thomist would see Paley as describing everything as artifacts.
What does Aquinas mean by saying an arrow must be pointed at a target? For illustrative purposes, consider an acorn. It has the adult oak tree as its end, as what it’s directed towards becoming. The acorn will not grow into a puppy dog, nor will it grow into a rosebush. In some way, then, the oak tree is the target that the acorn is pointed at. But if the oak tree is a target of the acorn, it must therefore be present in some way. The adult oak tree is not contained in miniature inside the acorn, so we can rule that out. We find oak trees in nature, but it would be true that if all oak trees and saplings poofed out of existence so that only the acorns remain, the acorns would still grow into oak trees. So existing oak trees are not what maintain the target of the acorn either. Aquinas rules out other explanations as well, such as a Platonic third realm. Another explanation is that a mind could hold the target of the acorn, but it could not be a human mind, as the acorn would still grow into an oak tree if there were no humans. (It would help here to understand that Thomas thought the human mind became/took on the form of a thing when it grasped it – not the brain, and not to rule out the brain’s role, but referring to a non-corporeal power that is part of how human reasoning works. Thomists still agree with this, but this isn’t a philosophy of the mind topic). And it could not be the mind of intelligent aliens, or angels, or anything contingent, for what is contingent need not have been nor need be in the future, and if the contingent beings never were or disappeared, acorns would still become oak trees. Aquinas, through better reasoning than what I’ve presented here, concluded that it must reside in a mind that is eternal and necessary, such that it could not not have been. He provides reasons that it must be one, that it must be actus purus, that it must be non-composite, that it must be omnipotent (the cause of all things possible and holding the ends of all things possible), omniscient, etc… (these later attributes I’m just summarizing. I haven’t gone over reasons for them here–i mainly wished to expand on the “arrow must be pointed at a target” line). Essentially, it resides in an eternal, unchanging, omnipotent, omniscient mind that is the source of all existence once all is said and done.
Now, it might be that one would wish to object to the particulars of my example, such as an acorn and an oak tree, but it would hold true for the teleology of anything, whether it be something like a lifeform down to molecules, atoms, gluons, and so on. They “behave” or “react” as they do because their ends/“targets”/what-they-do-when-X-happens is present in some way, that is, within the divine intellect.
I hope that explanation aids discussion. If I’ve misstated the Fifth Way, please correct me.
I don’t think the Fifth Way is the only reason Aquinas argued that the First Cause must be an Intellect, though it is intended to support it, and it’s another argument that is intended, like the cosmological arguments, to show that the eternal, omnipotent, yadda yadda mind must exist.