The popular science writer Matt Ridley is excited. The cause of his excitement is the final publication of the human genome, the string of 30,000 genes that constitutes our evolutionary endowment. The genome has, in his view, changed everything. The weary old debate between advocates of nature and nurture is about to be recast. "No longer is it nature-versus-nurture, but nature-via-nurture . . . To appreciate what has happened, you will have to abandon cherished notions and open your mind. You will have to enter a world where genes are not puppet masters pulling the strings of your behaviour, but are puppets at the mercy of your behaviour . . . My argument in a nutshell is this: the more we lift the lid on the genome, the more vulnerable to experience genes appear to be."

When I first read this paragraph, I too was excited. Could this really be a revival of the notorious theory of Jean-Baptiste Lamarck, according to which partners transmit to their offspring not just innate, but also acquired characteristics? A long letter to my parents, punctuated with expressions of reproach and self-exculpation, took shape in my mind.

The fantasy was soon cut short. Ridley is no Lamarckian. His talk of behaviour "influencing" genes or of genes being "vulnerable to experience" is just a sensationalist way of describing the well-known fact that genes require the co-operation of the environment to achieve their intended effect. Monkeys are predisposed to fear snakes, but they will not actually fear them until they see a snake scaring another monkey. Newborn ducklings instinctively follow their mother, but only if she makes the right kind of noises. Instincts, in short, do not operate in a vacuum; they presuppose a certain kind of environment. If that environment is lacking, the instinct will remain dormant.

The publication of the genome has provided Ridley with a convenient peg on which to hang these old insights. But it changes nothing essential. It was, indeed, Aristotle who first noticed that the tendencies governing life require a favourable environment to come to fruition. The 20th-century philosophical biologist Jakob von Uexkull built on this insight. He speculated that every species carries with it its own particular world of objects. In the world of a fly we find only "fly things"; in the world of a lion, "lion things". This world is, as it were, intentionally present within the animal; it is anticipated in its instinctual behaviour and in its very anatomy. This is why animals are so unhappy in zoos.

What about human beings? Philosophers such as Ernst Cassirer and Martin Heidegger seized on Uexkull's work because it seemed to illuminate, by means of contrast, the anomalous position of man. Flies have fly things. Lions have lion things. But what are our "things"? What is our "world"? The question evokes a peculiar kind of vertigo. Our world is filled with telephones, computers, minidisks. It is not a world on which we can repose, as on something stable and given, but one that we must continually make and remake for ourselves. It has no foundation in nature, or a foundation so remote that it is no longer visible as such. It is tempting to think of this alienation from nature as a uniquely modern condition, and perhaps it is felt most intensely in the modern world. But it is a central theme of all the great world religions. "The foxes have holes, and the birds of the air have nests; but the Son of man hath nowhere to lay his head."

Matt Ridley denies any such gulf between the human and animal worlds. His rehabilitation of nurture is strictly on nature's terms. The environment, in his account, is nothing more than a catalyst for tendencies already present within the organism. Its contribution is, so to speak, prefigured in the genes them- selves. And it plays this role indiscriminately for both animals and human beings. Ridley is thus unable to do justice to the sheer weirdness of our situation. He cannot take the measure of our "naturelessness".

Ridley acknowledges that human beings have something other animals lack, or have only in a very rudimentary form, and that is language. But he treats language as simply another natural endowment, on a par with elephant tusks or bat wings. If it is unique, it is unique only in the trivial sense that no other species possesses it. Like all other natural endowments, it possesses a genetic basis and an evolutionary function. Ridley speculates wildly - and, to do him justice, playfully - as to what these might be. A certain mutation in the Fox2 gene emerges as a possible basis for language. As for language's evolutionary function, gossip and social bonding are ventured as a feminist alternative to the conventional, phallocentric theory of hunting.

There is nothing wrong with this sort of speculation. But even if we could pinpoint the original evolutionary function of language with certainty, in what sense would this constitute an explanation of language? This is where the difference between language and other natural endowments emerges clearly. A genetic-evolutionary explanation of bat wings would tell us everything there is to know about bat wings. But a similar explanation of the origins of language would tell us nothing at all about language itself.

Language has long since transcended whatever evolutionary purpose it may once have served. Its purposes are now as various as the purposes of human life in general; it is used to abuse and to convince, to pray and to seduce. It has, in a sense, no purpose at all, because it is the medium within which all possible purposes are expressed. The impossibility of explaining language - or indeed human culture in general - with reference to prior, natural purposes is the substance of the old thought that man "transcends" nature. The acquisition of symbolism "transforms the whole of human life", explains Ernst Cassirer. "As compared with the other animals man lives not merely in a broader reality; he lives, so to speak, in a new dimension of reality."

Ridley has, it emerges, political reasons for rejecting the autonomy of human culture. It appears to him to grant states a unlimited licence to manipulate their citizens. Lenin and Trotsky took a keen interest in the work of Ivan Pavlov; the American behaviourist B F Skinner fantasised about a totalitarian society run according to his principles. A genetically grounded human nature seems, to Ridley, to offer the only secure guarantee against this kind of abuse.

I am unconvinced. It is surely the autonomy of human reason from all givens, both genetic and environmental, that constitutes the only safe foundation for individual liberty. Our genetic endowment offers only treacherous support, because it is in principle just as malleable as our external environment. With the advances in genetic technology, this possibility is rapidly becoming a reality. Ridley's worries about behaviourist dystopias are beside the point. No serious attempt has ever been made to subject human beings to Pavlovian methods. Genetic engineering, by contrast, is an imminent threat.

This threat is the subject of The Future of Human Nature, an intelligent short book by the German philosopher Jurgen Habermas. Habermas recognises that the new eugenics will be "liberal" as opposed to statist. It will take the form not of state-sponsored programmes but of private markets in genetic interventions. Perhaps this is why the new eugenics causes libertarians such as Ridley so little anxiety. Habermas is not so sanguine. Our political communities, he argues, are based on the assumption of symmetrical relations between free and equal citizens. Children are temporarily dependent on their parents, but this asymmetry is (one hopes) cancelled when they attain full autonomy in adulthood. The situation of a "genetically engineered" child would not be so happy. He would remain throughout his life in thrall to the intentions of his parents, intentions that he might reject yet could not reverse. He would be unable, so to speak, to identify with his own personality; it would remain for him a product, a manufactured item. The fundamental premise on which liberal societies are based would be destroyed.

To Habermas, the spectre of genetic engineering forces liberalism to acknowledge its ethical and religious origins. Liberalism is often assumed to be based on a purely procedural conception of justice, and is thus neutral between various ethical and religious systems. In fact, it rests on a "prior ethical self-understanding of the species". The monotheistic religions describe man as a "creature of God". Because God belongs to a different order of being, his creation "does not imply a determination interfering with man's self-determination". God "determines" man only in the very general sense of "enabling and, at the same time, obliging him to be free". But a very different kind of determination, a strictly causal determination, arises when God is supplemented by a human geneticist.

The metaphor of divine creation, in other words, expresses a belief in the ethical value of genetic randomness. Whether that belief can survive, once the metaphor is dispensed with, is a question that Habermas leaves moot.

Edward Skidelsky is an NS lead reviewer