St. Thomas's Fifth Way is:
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.
A standard question about design arguments is whether they aren't undercut by the availability of evolutionary explanations. Paley's argument is often thought to be. But Aquinas' argument resists this. The reason is that Aquinas' arguments sets itself the task of explaining a phenomenon which evolutionary theory does not attempt to, and indeed which modern science cannot attempt to, explain. In this way, Aquinas' argument differs from Intelligent Design arguments which offer as their explananda features of nature (such as bacterial flagellae) which are in principle within the purview of science.
Aquinas' explanandum is: that non-intelligent beings uniformly act so as to achieve the best result. There are three parts to this explanandum: (a) uniformity (whether of the exceptionless or for-the-most-part variety), (b) purpose ("so as to achieve"), and (c) value ("the best result"). All of these go beyond the competency of science.
The question of why nature is uniform—why things obey regular laws—is clearly one beyond science. (Science posits laws which imply regularity. However, to answer the question of why there is regularity at all, one would need to explain the nature of the laws, a task for philosophy of science, not for science.)
Post-Aristotelian science does not consider purpose and value. In particular, it cannot explain either purpose or value. Evolutionary theory can explain how our ancestors developed eyes, and can explain this in terms of the contribution to fitness from the availabilty of visual information inputs. But in so doing, it does not explains why eyes are for seeing—that question of purpose goes beyond the science, though biologists in practice incautiously do talk of evolutionary "purposes". But these "purposes" are not purposes, as the failure of evolutionary reductions of teleological concepts show (and anyway the reductions themselves are not science, but philosophy of science). And even more clearly, evolutionary science may explain why we have detailed visual information inputs, but it does not explain why we have valuable visual information inputs.