Aquinas’ Fifth Way represents a classic statement of the teleological argument qua purpose.  Like Aquinas’ first four ways (Summa Theologica 1, Question 2, Article 3) the argument is inductive and draws the conclusion that God exists a posteriori, following observations of characteristics of the natural world and specifically that all things seem to act for an end (Greek “telos”).  Also like Aquinas’ other ways, the fifth way cannot claim to prove God’s existence; as an inductive argument it is limited to concluding that God is the most probable explanation of the aspects of the universe named in the propositions.  Apart from that obvious limitation, Aquinas’ argument is beset by significant problems and, as this essay will demonstrate, fails to achieve its aim of being a good argument for God’s existence.

Aquinas’ fifth way can be expressed through the following syllogism

P1: natural bodies, which lack intelligence, act for an end

P2: whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence

C: Therefore some intelligent being exists by whom all natural things are directed to their end (and this being we call God)

The first proposition – that natural bodies which lack intelligence act for an end – could easily be disputed.  Might it not be that direction in natural bodies is more about how we see and understand them than about how they actually are?  Arguably, the human brain is hard-wired to see patterns and infer causation in the natural world.  Of course, without proposition one the whole argument will founder.

Even if this objection is dismissed as taking scepticism too far, proposition two – that whatever lacks intelligence cannot move towards an end unless it be directed by some intelligent being – is problematic.  Take a banana.  Creationists often cite it as an example of “intelligent design” in the universe.  The banana is a great size, shape, sweetness and colour for human consumption (even its skin features a reliable indicator of ripeness) it seems well designed for the end of being a tasty snack.  Yet to say that ignores the fact that neither the colour, nor the shape, nor the sweetness nor the size of the banana has anything to do with a divine designer – modern bananas have been selectively bred by farmers to have these attributes from parent plants which evolved to appeal to other animals such as monkeys who would spread the seeds of the plant by consuming its fruit.  While we can infer the existence of an intelligence from the brilliance of the modern banana in suiting the average human palate, to suggest that that intelligence is divine is a big step too far.  Even setting aside the modern banana in favour of the original “wild banana”, the “intelligence” that designed it is more probably evolution by natural selection than any God.  It seems that the second proposition “whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence” is seriously flawed and on this grounds as well could be said to fail in its aim of being a good argument for God’s existence. 

Further, Aquinas proceeds to use the analogy of an arrow and an archer to illustrate his claim that all natural things act for an end and so must have been designed to do so by an intelligent being.  The analogy is far from perfect and suggests a certain circularity in Aquinas’ reasoning.  As Hume’s character Philo observes, the selection of an analogy for the universe is far from neutral.  Scholars (including Aquinas) assume their own world-view in selecting something to compare the universe with and so by saying “the universe is like an arrow” or “the universe is like a watch” commit the fallacy of begging the question. If I compare teleology in the universe with an arrow then the suggestion of a necessary divine archer seems reasonable, yet if I compared the universe with a rock rolling down a mountain, which seems just as sensible an analogy – elements of the universe go through cycles, grow increasingly complex and make progress after all – then the inference that there must be an intelligent designer behind the process seems less obvious.  Rocks can roll down mountains as a result of non-intelligent actions, whereas arrows don’t tend to hit their marks randomly.  The “ends” which Aquinas claims that non-intelligent things act for could well be accounted for by natural processes such as evolution through natural selection, so it seems unnecessary to conclude that an intelligent designer, let alone the Christian God, exists.

Finally, Aquinas’ claim about direction and efficiency in the universe is a general one.  There are many instances of natural things failing to fulfil their apparent end or indeed not having an apparent end.  If God is the “intelligent designer” of the universe then what do the obvious inefficiencies in nature suggest about His competence, and (as Charles Darwin and John Stuart Mill both observed) what does the existence of beings whose end is to torment and destroy other beings say about His goodness? As Darwin wrote…

“I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.”

And as the Biologist JBS Haldane wrote…

“The Creator would appear as endowed with a passion for stars, on the one hand, and for beetles on the other, for the simple reason that there are nearly 300,000 species of beetle known, and perhaps more, as compared with somewhat less than 9,000 species of birds and a little over 10,000 species of mammals. Beetles are actually more numerous than the species of any other insect order. That kind of thing is characteristic of nature.” (“What is life?”)

As Hume’s character Philo concluded, the problems attendant on suggesting that God is the necessary designer of this universe, with all of its quirks and inefficiencies, are many.  Further, why one God?  Why not an apprentice God, a senile God… or one working as part of a committee? The final step in Aquinas’ argument, that of saying “this being we call God” is a giant leap and probably a leap too far.

In conclusion it seems that quite apart from the limitation of being an inductive argument, Aquinas’ fifth way fails to achieve its aim of being a good argument for God’s existence.  Aquinas’ first proposition can be questioned, his second seems to have no foundation in a post Darwin world, his analogy of the arrow and the archer is imperfect and so his conclusion that an intelligent designer-God must exist cannot be upheld. Nevertheless and despite its failure Aquinas’ argument retains value as an extremely clear statement of the teleological argument qua purpose, an argument which remains the most persuasive and which is probably the most widely cited reason for belief in God. Although the propositions fail to stand up to scientific scrutiny they seem reasonable, even undeniable to many people on an intuitive level.  On this basis modern scholars such as Alister McGrath and Richard Swinburne appeal to probability asking “which is more probable; that the apparent order and purpose nature is explained by chance and natural selection or that there is an intelligence shaping the process?”  They have more success in this limited endeavour than Aquinas had in seeking to advance a good inductive argument for God’s existence.

Further Reading

Aquinas’ Ways to God (New Advent)

Hume’s Dialogues Concerning Natural Religion