Does eye come before see?

Simon Blackburn wonders where 'function-talk' fits with Darwinism

Michael Ruse's book is one of the latest of a spate introducing the general reader to Darwinism - its origins, history, strengths and its place in contemporary science. It does so with a distinctive angle, however. Ruse teaches philosophy but, as well as making some philosophical suggestions, the book is notable for its history and, in particular, the way it brings to life so much of the 18th and 19th-century context within which Darwinism evolved. Ruse carefully takes us into the confused relationships between thinking in terms of evolution, adaptation, homologous structures, complexity and, finally, purpose and design. Kant, Georges Cuvier, Erasmus Darwin, William Paley, William Whewell, Huxley and others gravely take their places to the left and right hand of Darwin himself.

Ruse explains in his preface that his special interest is "a powerful mode of thinking - teleological thinking - that has traditionally had important implications not just for our understanding of the world we inhabit but also for notions of a creator or deity". This may ring alarm bells: most people who talk like this have some more or less hidden agenda of finding "intelligence" at work in the nature and evolution of life. They try to resurrect Paley and his watch. But, officially at least, Ruse has no such agenda, and certainly no patience with creationists or "intelligent design" propagandists who do.

Teleological thinking is thinking in terms of purpose, or of something for the sake of which something else is done, its "final cause". From Aristotle to Kant, biology was thought of as distinct from mechanics or physics just because of the central place it naturally gives to teleological thinking.

The reason why an organ has its place and its nature is because of what it is for, the function it serves in sustaining the life of the animal. The ear is for hearing and the eye is for seeing.

But, as Kant saw, it is a strain to reconcile this kind of thinking with the thinking of science. In physics, causation works from past to future. A future benefit cannot cause a present event. Tomorrow's eyesight cannot cause today's production of crystallins, the proteins largely making up the lens, nor the genes that orchestrate them. A preceding thought of this benefit might be causally efficacious, but that implies a benevolent and intelligent designer. So the options for biology seemed to be to go with Paley, finding the hand of God everywhere, or to give up characteristically biological thinking altogether.

Darwin can be hailed as the person who took teleology out of biology.

Adaptation through natural selection does the job for which it was needed.

Of course, much had to be added. Before Mendelian genetics, Darwinism offered no plausible mechanism for creating and sustaining the variations upon which natural selection needed to operate. Darwin himself had no plausible response to Fleeming Jenkins' devastating demonstration that if inheritance blends characteristics, as Darwin thought, then even advantageous variations will rapidly be diluted out of the gene pool. But by now we know the mechanisms, and Jenkins has long been answered. Darwin said that the eye "gives me a cold shudder", but we now know how evolution, several different times, used other older proteins to develop the crystallins that make these up. Evolution does it, we suppose, because it is an advantage to be light sensitive.

But looking at it a different way, Darwin can equally be hailed as the person who allowed respectable teleology into biology. In the post-Darwinian world we can see clearly how to distinguish function (respectable) from purpose (unnecessary). We can continue to say that light sensitivity is a function: it is that for the sake of which the eye exists, in the sense that it is the adaptive advantage that explains the superior evolutionary success of creatures that developed crystallins. And we can say all that without implying any purpose or design anywhere.

All this is familiar territory, summed up in Richard Dawkins' description of evolution as the "blind watchmaker". Ruse's book gives a distinct sense of being uncomfortable with it. It is as if he wants to pay full tribute to the scientific power of adaptive and evolutionary thinking while not quite letting go of the Kantian idea that we need something not quite scientific to reconcile us to it. For instance, he does not really like the account of biological function that I just sketched (due to Larry Wright) and he juxtaposes it with an account (due to Robert Cummins) that is silent about evolution and adaptation, and simply talks of the capacities of elements of a system - leaving it open how the molecular engineering brought about things with those capacities. But it is precisely this question that is demystified by Wright and is answered by stories of adaptation. Cummins deliberately leaves it open that it took an engineer to put together the complex in which all the capacities of the individual parts come together to make a system. In other words, he does not distinguish between radios and living things, any more than Paley did. It is not that this is wrong, but since we have more illuminating accounts of biological function, we should use them.

I do not think that Ruse disagrees with this, although he wants to retain the idea that thinking of "design" is a useful metaphor, and one whereby we see the biological world through a lens of values. I found this perplexing.

In principle, we can build, from the ground up, a story without either metaphor or value. Dawkins thus describes core Darwinism as "the minimal theory that evolution is guided in adaptively non-random directions by the non-random survival of small random hereditary changes". Surely it is not metaphorical, nor importing values of any kind, to assert that the evolution of the vertebrate eye was guided in an adaptively positive direction by the increased survival of small random changes in the genes that originally coded for heat-shock proteins, and which, through those changes, produced crystallins. The account might in principle be wrong, but it scarcely seems metaphorical, and it can be summed up by talking of function. Crystallins are adaptively positive because they enable their owner to fashion behaviours responsive to differences of light. That is what they were originally for and what they are still for.

If we go on and talk of purpose or design, we are importing a metaphor - and a dangerous one - but surely the right thing to do is avoid such language altogether. Ruse disagrees. At one point he says that "at the heart of modern evolutionary biology is the metaphor of design, and for this reason function-talk is appropriate". But that seems the wrong way round, the unconscious tug of the argument to design that he officially denies. At the heart of modern evolutionary biology are well-understood mechanisms of variation, adaptation and survival; it is for this reason that function-talk is appropriate, and metaphors of design should be avoided except for rhetorical purposes after dinner.

Ruse ends his book with a passionate call for a "theology of nature" that, instead of holding adaptation at arm's length, glories in it. It celebrates the complexity of the natural world and calls for "love and joy" at its myriad workings. He rhapsodises over the figure of the naturalist standing in the early Cambridge morning among dunnocks and cuckoos, or marvelling at the details of the butterfly's wing. Perhaps anticipating the response that we can all sing this hymn but that it goes better without the religion, he quotes Ernst Mayr, who told him in 1988: "People forget that it is possible to be intensely religious in the entire absence of theological belief." I am not so sure. Darwin was a peerless observer, but famously he was not deceived about the "clumsy, wasteful, blundering low and horridly cruel works of nature". This is the central trouble with "intelligent design" - even if you go in for it, all you get is a deity who designs worlds like this, and from that you can infer nothing of any interest. As David Hume put it, you cannot infer a designer who prefers good above ill, any more than heat above cold or drought above moisture.

Ruse shows some sympathy with the Panglossian idea that, given the constraints, perhaps this is the best of all possible worlds. But that is never lastingly popular with those of a religious temperament. The trouble for the believer is that it destroys any hope of a better hereafter. And for the rest of us it saps any will to work for the world's improvement. We do not want to sanctify the workings of cholera or tuberculosis.

Simon Blackburn is professor of philosophy, Cambridge University.