In Aquinas' own words:
"The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."What Aquinas is saying here, put simply, is that:
A) We observe in nature, things with no mind or intelligence, acting as if they have intention, purpose or goals.
B) It is a truth that only a being with a mind can truly have intentions.
C) Therefore, an intelligent being must be responsible for the intentionality we observe in nature.
There, in just a few sentences, Aquinas submits a rational proof that all of nature is intelligently designed! There is no need to argue, as most ID proponents do, that it is complexity and sophistication in nature that requires design. No, to Aquinas; even the rocks cry out "Design"! And this is true of nature everywhere we look! Everything we see, everything made of matter, has bits and particles within it whose job seems to be simply to maintain and sustain that very thing that they are a part of. There is absolutely no materialist explanation for this.
It's a beautiful thing. What's more, if one understands the full implications of Aquinas' simple proof, the designer must itself be outside nature. You can't cite nature to explain all of nature - so the explanation must be something separate from nature. Hmm... an intelligent being outside nature... I wonder who that could be?
Which leads me to the other attractive aspect of Aquinas Fifth Way: the fact that it points explicitly to God - not some other being who "may or may not be" God (as ID theory is so fond of saying.)
It's refreshing to be able to unequivocally say that all of nature (not just the complex stuff) is designed and that this designer must be God.
The further implications of this is that it reduces questions of evolution and abiogenesis strictly to scientific inquiry. Whether or not nature can produce a lifeform from non-living material has no implications philosophically or theologically. Either way, God was behind it. The same goes for the evolution of new biological types. It's all design, all the way down.