It is not uncommonly thought that Aquinas's fifth proof for the existence of God – as it is found in the Summa Theologiae – is nothing more than a species of the Intelligent Design argument. For example, a quick perusal of the Wikipedia entry for “Intelligent Design” informs us that “the argument from design, the teleological argument which has also been called the ‘argument from intelligent design’, has been advanced in theology for centuries. Thomas Aquinas presented it in his fifth proof of God's existence.”Footnote ¹ The reader is, thereby, left with the impression that Aquinas's Fifth Way is little more than an archaic predecessor of the later theories of William Paley, William Dembski, and Michael Behe.Footnote ² This is unfortunate because it naturally leaves the impression that objections made to Intelligent Design arguments are also valid objections to Aquinas's Fifth Way. In this short essay, I wish to explain why this is not the case.

There are of course a variety of Intelligent Design arguments. William Paley is usually credited with getting the ball rolling with his analogy of the watchmaker. In his book, Natural Theology, first published in 1802, he compares the complexity of the natural order to the complexity of a watch and concludes from this that both must, in their own orders, have a Designer.

More recently, William Dembski has developed the notion of ‘specified complexity.’ This is the idea that living things demonstrate a complexity that is specified. By specified is meant that living things manifest reoccurring patterns in their constitution: in DNA and molecular structures, for example. Hence there are patterns and these patterns are complex. Dembski seeks to show that this cannot be accounted for by chance and so reveals the handiwork of a designer.Footnote ³

Also in more recent times, Michael Behe has proposed the concept of ‘irreducible complexity.’ Behe notes that even simple organisms demonstrate significant complexity. He asserts that for these organisms to have evolved by chance mutation and natural selection is extremely improbable. Hence the more reasonable explanation for this complexity is a Designer. By way of analogy he invites us to consider the construction of a mouse trap.Footnote ⁴ The classical mousetrap is made up of five components – a board, a spring, a clasp, a bar, and a hoop into which the bar is placed to hold down the clasp. Behe points out that each of these elements only has utility as part of the whole and, therefore, depends on the simultaneous perfection of the other four parts. Analogously, the argument runs, the theory of natural selection would require each part of a natural organism such as an eye or a kidney to be perfected at the same time. If just one of the many parts was not already fully developed all the others would remain useless and not be selected by the mechanism of natural selection. This simultaneous perfection is so unlikely that the positing of a Designer is more rational.

The main criticism offered by opponents of this argument is that the kind of complexity Behe is thinking of can, contrary to his assertion, be accounted for by evolution, understood as chance mutation and natural selection. Opponents argue that neo-Darwinian evolutionary theories are not claiming that the precursor of an organism is a less perfect version of the same organism, but some completely different organism perfect for its different function. For example, Kenneth R. Miller, writes that:

Miller goes on to claim that we can find just such cases in nature. Smaller components of a large organism do have utility on their own. The example he gives is that a subpart of a bacterium's flagellum has utility as a tool used by other bacteria to inject poison into cells. Of course, this hardly definitively refutes Behe because we would need to find a complex organism where all its subcomponents have utility other than when combined. Nonetheless, it at least hints at possible difficulties in Behe's Intelligent Design argument.

Moreover, we would have to admit that Behe's theory (along with Dembski's) is founded on ignorance. The logic of Behe's argument runs that because we do not know how a more complex organism evolved from a less complex, therefore it is impossible and so the special intervention of an Intelligent Designer is required. But, obviously, building an argument for the existence of God on ignorance is precarious. It has the atmosphere of making oneself a hostage of fortune since perhaps new scientific discoveries will reveal to us how these complexities might result from evolutionary processes. In this way, an argument built on irreducible complexity or specified complexity is often pigeon-holed as an argument for a “God of the gaps”.Footnote ⁶ God is posited as a theory that enables us to fill various gaps in our knowledge of how nature works – gaps liable to become ever narrower with new scientific discoveries.

Now, compare this to the Fifth Way of St. Thomas. This proof is so wonderfully short that we can afford to quote it in full:

Aquinas begins by noting that things act towards ends. By this, he means that it is a simple observable fact that each type of natural thing acts in the same regular way. So apple trees (if they are not diseased) produce apples, the moon revolves in a regular path around the earth, rocks fall towards the ground when dropped, fire burns. It is not that it is just statistically likely that these different things will act in these particular ways; rather it is that these things by their very nature act in these ways. Using the classical philosophical terminology we could rephrase all this by saying that Aquinas is asserting that from observation of things around us we observe that they display final causality.

The second premise is that nothing acts for a goal unless moved to that goal by something which knows what the goal is. Of course, this leads to the conclusion that there must be an Intelligence which accounts for all these irrational creatures acting for goals that they cannot possibly be conscious of themselves. At first sight this second premise might appear a little obscure. However, what Aquinas seems to be saying is this: the final cause – the goal – is the cause that is driving the activity of the thing that is tending towards that goal. Hence, the whole development (and so activity) of the apple tree is ordered towards the goal of producing apples. However, the apples themselves are not yet really present. Thus, we seem to have a situation where a non-existent cause is causing something. This, of course, cannot be true. The fact is that for a cause to exert causal power it must be present in some way. The conclusion, therefore, is that the final cause is indeed present – it is present in the mind of God.Footnote ⁸

The first thing to note about this argument in comparison to the Intelligent Design argument is that it does not depend at all on the existence of complexity. As Edward Feser notes, even if the universe consisted of one simple atom, the fact that this atom always acted in a regular way would be enough to observe final causality and hence sufficient material to mount the argument of the Fifth Way.Footnote ⁹ Therefore, following Aquinas's approach, we do not get embroiled in arguments about the likelihood of this or that level of complexity coming about by chance. Such arguments are nearly impossible to prove conclusively one way or the other and this leaves the whole debate hanging in the air. Ultimately, this is because the Intelligent Design argument is not a rigorous philosophical proof but a probabilistic argument. It argues that it is so improbable that observed complexity can come about by chance that it is more probable that there is a designer.Footnote ¹⁰ In contrast, the Fifth Way is not playing with probabilities: it is a rigorous argument in the sense that if the premises of this argument are correct, it is not probable that there is an Intelligence, there simply must be one.

The second thing to note is that while the Intelligent Design argument is a possible target for objections that chance and natural selection are the cause of complexity, the Fifth Way is not nearly so easily disturbed by the idea of evolution. Consider: what if someone were to say that the goal directness of irrational things is the result of evolutionary forces – in the same way that chance and natural selection are the cause of organic complexity? In fact just this claim is made in another encyclopaedia: The International Encyclopaedia of Philosophy. There we read:

The answer to this is that final causality can never be explained by chance and natural selection because both chance and natural selection presuppose final causality. After all, chance is not an effect that is uncaused, but rather the confluence of causes that are not necessarily associated with each other. Aquinas gives the example of two servants who meet in the market by chance, but of course each is there because they have been sent and so a cause can be assigned to their meeting each other.Footnote ¹² But if chance events are still caused events this means that chance depends on the action of pre-existing causes which already display final causality (goal directedness). For example, if certain environmental factors cause gene mutation (a mutation that makes an organism either better or worse suited to its environment) this is because those environmental factors have certain causal properties. So if mutation occurs because of exposure to some chemicals it is because those chemicals already have determinate ways of acting (final causality) in contact with other materials.

What is true of chance is also true of natural selection – goal directedness is presumed and not explained. If some chance mutation proves to be beneficial it is because it occurs within an environment that is stable enough for that change to be selected: but this stability is nothing more than things having final causes. So, for example, if historically a certain mutation led to the existence of chlorophyll; that this new reality was beneficial to any organism presupposes that many things in the environment acted in a uniform stable fashion. For example it presupposes that fused hydrogen atoms give off radiation in the formation of helium which is the origin of light from the sun. Without this stability – of which final causality is the linchpin – chlorophyll would have no lasting benefit and would give no edge in the fight for survival to those organisms possessing it.

The upshot of this is that the two pillars of evolution (chance and natural selection) presuppose the existence of final causality and, therefore, can hardly be the ultimate explanation for the existence of final causality. Not so with complexity: it might well be that chance does indeed cause complexity, however statistically improbable this might be.

And at this point we meet yet another problem with the Intelligent Design argument which is, to my mind, the most serious. Intelligent Design arguments, like those mounted by Behe and Dembski, concede a premise that should never be conceded and that Aquinas would certainly not concede. The whole logic of the argument is built upon the premise – shared by materialists – that if chance is a significant element in the evolution of species then God's involvement in directing the development of species must be discounted.Footnote ¹³ However, this premise is not philosophically sound. As I have already pointed out, by a chance event we do not mean an uncaused event but merely an accidental confluence of several causes. But if God is the ultimate source of all being, including those beings which are causes (as Aquinas's Third Way proves and Fifth Way assumes) then chance events hardly escape His providence. As the Angelic Doctor reminds us:

When applied to evolution, this means that God might just as easily cause events (like the development of species) by chance as by regular predicable sequences of causes. This at least shows that the premise ‘if chance no God’ is unsound. The point is, of course, that the Fifth Way, yet again, makes no such concession while Intelligent Design does.

Finally, the Intelligent Design approach parts company with Aquinas's vision of the universe in another very significant way. As we have seen, Paley and Behe compare complex organisms such as eyes to man-made machines such as watches and mousetraps. From the design complexity of the later they infer that the former must also be designed. It is certain that Aquinas would balk at comparing eyes to watches. The reason is that, for Aquinas, watches are artefacts whereas an eye is a natural substance (even if an incomplete one).Footnote ¹⁵ Artefacts are created out of different natural substances – such as copper, steel, and crystal – but they are the accidental (even if planned) arrangement of parts with a purpose imposed from without rather than arising from the very nature of the things it is made out of: copper has no natural tendency to tell the time whereas an eye has a natural (inbuilt) tendency to see. This means that, from Aquinas's perspective, the starting premise of Intelligent Design is very questionable, since eyes are simply not like machines in a very significant respect.

A dubious starting premise is, of course, likely to shake our confidence in the whole argument.Footnote ¹⁶ Furthermore, there are at least two important limitations built into the premise that natural substances like eyes can be considered as similar to artefacts like watches. The first is that this argument cannot lead us to Anselm's God, namely “that than which nothing greater can be conceived.” At best it can lead to a very great intelligence which uses pre-existing materials to construct complex organisms. This intelligent designer is higher up the ladder of cleverness than we are, but it is still on the same ladder: the difference between it and us is a matter of degree and not a matter of type. He is, in the words of C. F. J. Martin “an impostor disguised as God, a stern, kindly, and immensely clever old English gentleman, equipped with apron, towel, square, and compasses.”Footnote ¹⁷

In short, the Intelligent Design argument cannot get us to a Creator of the Universe, but only a maker. After all, the watchmaker is precisely that: a watchmaker and not a watchcreator. The watchmaker assembles a watch from materials provided by another. The Intelligent Designer likewise assembles complex organisms from natural substances supplied by, in this approach, we know not what. But the Creator is precisely that who brings things from nothing.

The Fifth Way, however, does get us to a Creator and not just a maker. This is because it relies on the idea of substances with inherent goals or, as Aquinas would have it, on things having both formal and final causes. But any substance that has a goal (a goal that is not itself) has potency to perfection: a perfection that is realized in attainment of that goal. But anything that has potency is not pure being (self-subsisting being) and so must receive its existence from something that just is being.Footnote ¹⁸ Hence we are inexorably led back to not just to a Mastermind Champion but Pure Act: the God of Moses – I AM WHO AM (Exodus 3:14–15).

The second fallout from the mechanistic vision of reality underpinning the Intelligent Design approach is that it cannot lead us to a provident God. The watchmaker makes his watch and then sits back and admires it as it methodically carries out its task of telling the time, now wholly independent of the watchmaker himself, who as far as the functioning of the time-piece is concerned might even have retired, nay died. So, Intelligent Design gets us as far as Deism at very best.Footnote ¹⁹ The Fifth Way argument is constructed on the idea that every act of efficient causality – every effect brought about by irrational agents – is only explainable by the action of an Intelligence here and now.Footnote ²⁰

It is not that there is absolutely no overlap between the Intelligent Design argument and the Fifth Way, it is clear that both of them argue for the existence of an Intelligence that accounts for natural observable phenomena. However, beyond this point, resemblance fades rapidly. The Fifth Way is an attempt at a rigorous proof for the existence of God, Intelligent Design is probabilistic. The Fifth Way could be applied to the simplest of worlds, Intelligent Design demands complexity. The Fifth Way is not susceptible to objections of chance and natural selection, whereas the Intelligent Design argument is highly vulnerable and, more alarmingly, accepts that false premise that chance excludes the activity of God. Finally the Fifth Way demands the existence of an all powerful, all actual, provident God, while Intelligent Design leads us to a superior Intelligence, one which, in the words of Feser, you would not want to play Trivial Pursuit with, but which is a far cry from the all powerful Creator God of the great monotheistic religions.Footnote ²¹ Moreover, unlike the God of the Fifth Way, this intelligent being interacts with the universe only at key moments.

Seen in this light, the whole thing reads like a comic play in which there are putative brothers (Fifth Way and Intelligent Design) who are thought to be blood brothers by a complete stranger (neo-Darwinist materialism). The brothers are superficially alike in external appearance and share the same faith. However, as the play progresses and more and more is revealed about the background of the brothers it becomes apparent to the older brother (the Fifth Way) that they are not brothers at all. However, the younger brother (Intelligent Design) seems to be unaware of all this or at least unable to accept this difficult news. Moreover, further investigations reveal that the stranger is actually a cousin of the younger brother. Ultimately, the whole comedy has a tragic edge to it because neither the younger brother nor the stranger can be brought to recognize their common blood, nor that the older brother is really the odd man out. Further more, the inability of the younger brother to come to terms with the shocking news and his continued desire to associate himself with the older brother hinders the latter making his own way in the world.

Martin goes as far as to say that “like other bad arguments, its [Intelligent Design's] defeat and death has left it to wander the world like a ghost, oppressing the spirit of those who are looking for other and better argument.”Footnote ²² I think this is going a little too far! Let me clearly state that while I have pointed out many potential problems with the Intelligent Design argument, I am not claiming that it is in fact wrong to claim that the complexity of organisms indicates the existence of a Divine Designer. All I am pointing out is that it can never be anything other than a probable argument: by its very form it will never be a strict proof. Moreover, and more importantly, all the objections mounted by radical atheists against Paley, Dembski, Behe and such like arguments of design are water off a duck's back when they are turned against Aquinas's fifth argument for the existence of God.Footnote ²³ I suggest that this more ancient pillar would remain unshaken were science ever to send the Intelligent Design argument crashing to the ground.