40. The short form of the fifth way is:
A. Many natural things that lack knowledge nonetheless act to achieve the good always or the greater portion of the time.
B. From this we conclude that they act for an end, and that they achieve their end not by chance but by intention.
C. But whatever lacks intelligence cannot tend toward an end unless directed by something intelligent.
D. Therefore some intelligent being exists by whom all natural things are directed toward their end.
41. A version of the argument from design was in former days one of the most obvious and compelling arguments for God's existence. One hardly needed to work out the metaphysical details to convince of the existence of God someone who had stood in awe beneath the sphere of fixed stars and marveled at the regularity of their motion, or stared out into the sea by night as the tide receded and returned. The mind leaps readily from these things to the divine.
42. But, the story goes, Darwin has forever destroyed the Fifth Way, by showing how chaotic processes can lead to the emergence of order in natural species. This is partially correct, though other factors have contributed to the decline of the argument: the technological world has separated the city-dwellers of the world from nature, blotting out the stars at night, and pushing back the wilds, so the experience of awe (or at least, awe before the natural order of the universe) is no longer present in our daily lives.
43. Still, even if it is conceivable nowadays that there should be emergent order without a designer, the Fifth Way reaches deeper than the appeal to a designer. The argument from design is, in the forms we know today, insufficient to prove the existence of God, even without Darwin. The mere existence of complex forms that function well does not establish the existence of a transcendent God: the designer might be just another thing, a demiurge fashioning these forms and mechanisms out of pre-existing matter.
44. Thomas’s thought runs deeper than this, however, and so we can read into his fairly straightforward statement of the argument a fuller line of thought that depends not just on the appearance of design in nature, but the universality and nature of final causes, across all existing things.
45. The key to reading the Fifth Way is to see it as a reflection of the first. The First Way looked at the motions of the natural world, observed that every motion requires a mover, and asked “What governs and sustains these natures? What moves all things to act as they act?” The Fifth Way looks at the motions of the natural world, observes that things tend to act toward things, and asks “What directs these tendencies? What gives tendency to nature in the first place?”
46. We observe in the world a multitude of natural things: things which consistently act and cease to act according to inner principles. The ball comes to rest when its momentum has been exhausted by friction and its tendency toward the center of the earth has been counterbalanced by the resistance of the soil to compression. In every individual thing’s act and cessation from act we observe the operation of these principles, and in each case the terminus is some state of rest.
47. The question, then, is what governs the motion and rest of natures? Why do things come to rest? This question is applicable not only to natural substances and unintelligent life forms, but also to humans. What fixes the ends of nature, and directs each particular action toward that moment of fruition in which the agent rests? If there is purposiveness in nature, and if the natural motions of things tend toward a more perfect harmony, there must be a reason for this. Or, to apply the thought to something living: if there is purposiveness in the oak tree, such that its springtime foliage is really an end and perfection, not relatively but absolutely, then there must be a reason why the nature of the parts tends toward the perfection of the whole. There must be a reason why nature tends toward perfection at all.
48. This question shows us why the Fifth Way cannot point toward a demiurge: if there is a designer which directs nature toward fruition, then it must not itself be the sort of thing the nature of which is directed toward fruition. In other words, the designer of the Fifth Way cannot change, and cannot be distinct from its own perfection.
49. St. Thomas applies this reasoning specifically to unintelligent natural substances, because it is easiest to see that their acts, though suggesting intention, are not in themselves intentional. However, since Darwin, the limitation of the argument to unintelligent substances is no longer particularly advantageous. We can reframe it as follows:
50. In nature we observe that each thing tends toward its proper end and perfection, and that things tend to do this effectively, all or most of the time. But within things, the individual nature does not intend, but only disposes. This is true not only of unintelligent agents, but also of humans. Though particular desires and intentions may align with the working out of nature, those desires and intentions are themselves determined by the nature of which they are an expression. Therefore we must ask why it is that nature is directed at all. But to direct or design belongs to intelligence, which lays ahead of things their intended courses and brings things to pass harmoniously. Therefore if there is direction in nature, there must be some intelligent being which is the source of nature’s direction, and which is itself undirected. This is what we call God.
51. The argument is not watertight, but draws on the reader’s presumed belief that natural things are actually directed toward ends. Implicitly this belief is nearly unavoidable. This is, for example, the way metaphors are framed to explain biological processes, almost universally. Appeals to the real “superiority” of surviving species are endemic in discussions of Darwinian evolution. Thus social darwinism, etc. However, explicitly the claim that nature tends toward perfection is readily denied by educated people. Thus the Fifth Way is out of season for a while. Its day will come again.