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Gaia is an idea about life and its relationship with the planet. By carrying the name Gaia (first suggested for it by the novelist William Golding) it has resonance in poetry, mythology and religion. The idea, and the name of the idea, sometimes tug in different directions. Whereas the first has proved a fruitful scientific hypothesis, the second has been the basis for a new, and also very old, philosophy or even religion for our times.

Gaia in her present form sprang from the creative mind of the scientist James Lovelock. Although the idea had germinated over many years, his first book, Gaia, now republished in its original form, did not come out until 1979. In his own words, he wrote as a story-teller, and gave poetry and myth their place along with science. He then wrote a second book, The Ages of Gaia, first published in 1988 and now republished in revised form, in which he placed the emphasis on the science. There he explored the notion of geophysiology as a different expression of the same thought. He rightly pointed out that before geophysiology could be accepted into "the ample but straight laced bosom of science, it must be scientifically correct". The republication of these two books is an opportunity for looking again at what Gaia, or geophysiology, is about, and the impact the idea has had on both scientific and popular thinking.

Definitions are vital in this debate. They are more difficult because Lovelock's view of Gaia has changed somewhat over the years. The idea that the animate and inanimate worlds shade into each other has a long history. Even the idea that the animate may in some sense control the inanimate was given scientific respectability as long ago as the end of the 18th century. Perhaps the most useful and succinct definition was set out in a paper Lovelock published with his colleague Lynn Margulis of the University of Massachussetts in 1974. There they stated that Gaia theory is about the evolution of a tightly coupled system whose constituents are the biota and their material environment, which comprises the atmosphere, the oceans, and the surface rocks.

Had the debate been conducted in these sober sided terms, reactions to it might have been different. As things turned out, Gaia appeared, in spite of disclaimers, like some kind of sentient being who regulated the physical and chemical condition of the surface of the earth, the atmosphere and the oceans to make them fit and comfortable for living organisms. The choice of the name of a goddess, or earth mother, for a scientific hypothesis rather than an indigestible acronym had all too rapid appeal. But it put off people who did not fancy goddesses, and increased inevitable suspicion of an idea that brought together thinking from many disciplines in a range from astronomy to geology, and from paleontology to biology.

At one extreme there were those, in which most scientists were once included, who scarcely reckoned with the influence of life on the physical mechanisms that have made the earth what it is. According to them life had always been able to adapt itself to change in such mechanisms, sometimes by luck and sometimes by serendipity. At the other extreme were those who saw life driven by purpose in the way that our ancestors saw God's design in the creation of mankind and a nature subservient to it. Somewhere in the middle of the argument came those for whom Gaia could be a collective organism like an ants' nest or the jellyfish known as the Portuguese man of war: a distinct creature whose parts were both interdependent and independent, and which responded to its physical environment while in some cases helping to create it.

How did Gaia first come into Lovelock's mind? It began with his work for the United States National Aeronautics and Space Administration (NASA) on the question of whether there was life on our sister planet Mars. By examining the evidence for the atmosphere of Mars, he concluded that it was in a state of chemical equilibrium. This was in marked contrast with a top-down view of the atmosphere of the earth, which was in a state of disequilibrium. Such disequilibrium could only be caused by living organisms. From this he concluded there was no life on Mars, or indeed on Venus either. Expensive space probes had been rendered unnecessary.

This led him to look anew at the condition of the earth. Going back to its earliest days, Lovelock was able to work out the increasing influence of living organisms in creating the surface of the earth we know. A central piece of evidence for him was the relative stability of the temperature of the atmosphere since life began. During that time the sun's total luminosity - its output of energy as heat and light - has increased by as much as a third or even more. Yet the fossil record indicates that the earth's temperature has not varied by more than a few degrees Celsius. Knowledge of what has happened on other planets would have suggested otherwise. Lovelock concluded that the most likely explanation lay in the work of living organisms, which unwittingly strove to keep temperature as near to a constant, or in biological terms homeostasis, as possible.

Obviously such organisms could not adjust themselves to everything. Some nearly poisoned themselves to extinction by causing oxygenation of the atmosphere. Others were profoundly affected by the successive impacts of planetesimals, asteroids and bolides, such as that which probably brought to an end the long dominance of the dinosaurs. But in each case the continuity and stability of life itself was maintained or restored. Of course in the end organisms will be unable to moderate a steady increase in the sun's luminosity (around 1 per cent every 100 million years), and life will be extinct long before the sun becomes a red giant star some millions of years hence.

To respond to the criticisms of scientists and others, Lovelock set up a computer model that he labelled Daisyworld. Under this hypothesis he postulated a planet of black and white daisies in orbit around the sun that was becoming slowly but steadily more brilliant. At first, as the sun was cool, not many daisies would grow. As the sun became warmer, patches of both black and white daisies would sprout and flourish. By absorbing the heat black daisies would have the effect of raising local and eventually global temperatures. By reflecting the heat white daisies would cause the earth to become cooler again. Thus within certain limits, and in accordance with Darwinian principles, daisies would act as a planetary thermostat.

It is not easy to test a hypothesis of the scope of Gaia. But in some respects the microcosmic world of bacteria offers the best examples. As has been pointed out, the earth's bacteria form a complete regulatory system, which has the specific effect of stabilising the percentages of reactive atmospheric gases and the general result of keeping the earth habitable. Lovelock well shows how bacteria have been doing this since the beginning of life, and examines the implications of the role bacteria might play in the eventual colonisation of Mars or other objects in the solar system.

Obviously the use of words has greatly bedevilled the debate. Lovelock's references to the earth as an organism or a living organism raised objections from those who said that the prime characteristic of organisms was that they could reproduce themselves. The reply, that Gaia was in a sense immortal, and might anyway reproduce itself on other planets, was really beside the point. As a tightly coupled system whose constituents are the biota and their environment, Gaia must be sui generis, and does not have to justify this or that label. If it has to have one, "superorganism" is more plausible: it brings in the notion of the infinite interconnectedness of life, and suggests a Gaia in which living organisms from bacteria to elephants, from mushrooms and hollyhocks to termites, fit with an ever changing physical environment within a cybernetic feedback system.

Even the sternest critics can, I think, agree that the concept of Gaia has been a productive intellectual tool, and that research arising from it has contributed to understanding of the relationship between animate and inanimate nature. It has, for example, led to appreciation of the role of dimethyl sulphide in condensing water vapour to form clouds, and in distributing sulphur over land. It has suggested a new approach to the perennial problem of the saltiness of the sea. It has looked anew at the place of the ice ages in recent climatic history: Lovelock believes that ice ages should now be regarded as the normal climatic condition, and that interglacials, of the kind humans have enjoyed for the past 12,000 years, are a product of exceptional circumstances (in more picturesque terms Gaia is having a fever) that should end when - among other things - enough plankton pull enough carbon dioxide out of the atmosphere.

For myself I draw two main conclusions. Life is a planetary scale phenomenon. Through the normal mechanisms of evolution, it unwittingly helps to regulate the physical environment, and to maintain it within the bounds necessary for its own survival. Second we cannot consider the evolution of species separately from the changes in their environment, and within certain limits the environment is profoundly affected by life in all its multiplicity.

Where in all this is the human role? Lovelock underlines that Gaia has no particular tenderness for the human or any other species. The microbial world is probably the single most important element in the maintenance of life on earth. But humans, like any other species with runaway numbers, can do a lot of damage to other species and even affect the good functioning of the life system (all species are equal, but some are more equal than others). Lovelock is particularly concerned about our destruction of biodiversity, and interference with atmospheric chemistry. However temporary in geological terms, global warming could increase the planetary fever of the interglacial with unpredictable consequences. If Gaia can survive the periodic impact of planetesimals from without, it can surely survive the abuses wrought by one animal species from within. But it could change to our disadvantage as a result.

Lovelock is an inventor as well as a scientist, and sees problems in practical as well as general terms. He has also contrived to achieve and cherish an independence that enables him to say what he thinks without fear or favour from those who manage research grants. Ours is the age of specialists, whether scientists or others, who inhabit little boxes and rarely like to lift the lid. Lovelock lifts lids wherever his mind ranges, and brings others, sometimes blinking, into the light of a world where the big questions need more than ever to be debated. His written style tends, in his own words, to be like the pattern of a mosaic that makes sense from a distant view. At the same time he is the master of a penetrating phrase. For example, he asks the rhetorical question whether humans should or could take on the task of managing the planet and looking after its health. His response is that he would sooner expect a goat to succeed as a gardener than expect humans to become stewards of the earth. I fear he may be right.

A measure of the impact of the Gaia hypothesis as expressed in The Ages of Gaia is that some of it already looks like a truism. We have accepted it unawares. The science of it is already proliferating. It remains an inspiration for those for whom the old religions have lost their power and plausibility. I see no necessary conflict between the science and the inspiration. Each can become charged with new meaning. Lovelock has done great service to both.

Sir Crispin Tickell is warden, Green College, Oxford, where he organised a scientific conference on Gaia in 1994.