At one time ranked among Britain's most influential scientists, the crystallographer J D Bernal (1901-71) recognised no limits to the power of science. A lifelong Marxist and recipient of a Stalin Peace Prize, Bernal believed that a scientifically planned society was being created in Soviet Russia; but his ambitions for science went far beyond revolutionising human institutions. He was convinced that science could bring about a transformation in the human species - a planned mutation in which human beings would cease to be biological organisms.

Bernal's dream was that human society would be replaced by what Philip Ball describes as "a utopia of post-human cyborgs with machine bodies created by surgical techniques". Further ahead, Bernal envisioned "an erasure of individuality and mortality", in which humans would cease to be distinct physical entities. In a passage Ball cites from his book The World, the Flesh and the Devil: an Inquiry Into the Future of the Three Enemies of the Rational Soul (1929), Bernal looked forward to this apotheosis:

Consciousness itself might end or vanish in a humanity that has become completely etherealised, losing the close-knit organism, becoming masses of atoms in space communicating by radiation, and ultimately perhaps resolving itself entirely into light.

Bernal's strange fantasy shows how science can be a channel for ideas that owe more to mysticism than to dispassionate study of the world. The idea that human beings might shed the mortal flesh to enter a realm of deathless light harks back to the ancient religion of Gnosticism, while the belief that science can animate dead matter and fashion artificial human beings renews the visions of the medieval alchemists. The fact is that science has often been used as a channel for myths in which human beings acquire magical powers. Predictably, this has generated counter-myths in which science is demonised as a semi-diabolical force.

Ball's aim in Unnatural is to bring clear thinking to bear on the question of what science can and should aim to be, and it would be hard to find a more lucid and reasonable guide to contemporary controversy about the use of science to create life. Hidden underneath the sometimes bitter controversies surrounding IVF, embryo research and human cloning are ideas inherited from thousands of years of myth-making. "Natural" and "unnatural" are not scientific categories. Heavily freighted with ideas about what is good and right, they embody judgements of value that express immemorial hopes and fears. Ball uncovers these mythic traces and shows how they continue to shape our understanding of the life sciences and the new reproductive technologies these sciences have made possible. In light and graceful prose that is a pleasure to read, he provides an absorbing cultural history of "anthropoeia" - the project of "making people".

A striking feature of Ball's account is the ease with which it moves between science and the arts. It is refreshing and instructive to have

detailed discussion of recent advances in stem-cell research alongside descriptions of the fictions of Balzac, Poe, Huxley and Wells. Even more impressive is Ball's range of reference, which moves from Greek prehistory through the golems and homunculi of medieval Europe, through the unhappy ogre pictured in Mary Shelley's Frankenstein, up to the muddled obscurantism of the Bush administration's policies on stem-cell research.

If Ball provides an illuminating cultural history of the myths surrounding the attempt to use science to make people, his attempt to demystify the contemporary debate is less convincing. He argues that those who oppose the project of "making people" deploy beliefs about what it is to be human that can only be explicated in religious terms, writing: "The notion of a soul can no longer be considered intellec­tually respectable, and can certainly play no part in discussions about what constitutes life or personhood, or how we should think about the status of the human embryo." Maybe so, but opponents of people-making are not the only ones who invoke an idea of the soul. So do ardent supporters of the project when they propose using science to transcend the human condition. Bernal viewed religion with contempt, yet in thinking of "the rational soul" as a spark of consciousness imprisoned in the material world, he was reproducing a conception of human nature that is quintessentially religious. Among evangelists for "scientific humanism", the notion that rationality is the essence of humankind is an idée fixe. But it has no basis in science.

Aiming to demythologise our thinking about humankind's place in the scheme of things, Bernal reproduced an ancient myth of salvation. Ball is far more balanced, but his exercise in demystification seems to me to be similarly self-defeating. Seeking to purge us of myth, he proposes that we approach the world without assuming that what is "natural" is good. In effect, he is advocating that we embrace a rigorous form of scientific naturalism - a method of inquiry that makes no metaphysical assumptions about the goodness or otherwise of the environment in which the human animal finds itself. Science yields knowledge of how the world works, Ball maintains: it is up to humanity to use that knowledge to improve the world.

The trouble is that "humanity" is also an idea shaped by myth. In an interesting discussion of classical Greek ideas of nature, Ball notes that, from the late 5th century BC onwards, philosophers began to view tekhne - the art of making things - "as a means of coercing nature, by force or even by 'torture', so as to gain mastery of it and transgress its boundaries". Aristotle portrayed tekhne as "a kind of handmaid to nature, helping to bring it to a state of greater perfection". In this Greek conception, everything had a purpose. Even the universe was striving towards perfection, and the role of human beings was to assist in the realisation of that purpose. This view of the world ceased to be viable when Darwin removed the idea of purpose from biology, leaving only natural selection operating against a background of random events.

Despite Darwin, the classical Greek view of things has not been abandoned. The idea that humankind has a special place in the scheme of things persists among secular thinkers. They tell us that human beings emerged by chance and insist that "humanity" can inject purpose into the world. But, in a strictly naturalistic philosophy, the human species has no purpose. There are only human beings, with their conflicting impulses and goals. Using science, human beings are transforming the planet. But "humanity" cannot use its growing knowledge to improve the world, for humanity does not exist.

No doubt rightly, Ball cautions against basing our thinking on unexamined myths. He seems not to have noticed that the idea of humanity intervening to improve nature is just such a myth. Clearly, thinking about the human animal in rigorously naturalistic terms goes very much against the grain. Could it be that such a way of thinking might be - dare one say it - somehow unnatural?

Unnatural: the Heretical Idea of Making People

Philip Ball

The Bodley Head, 384pp, £20

John Gray is the NS's lead book reviewer.

His latest book is "The Immortalisation Commission: Science and the Strange Quest to Defeat Death" (Allen Lane, £18.99)