For no good reason, I have a strangely vivid visual recollection of John Gray from several years ago. I had just boarded a bus, in Cambridge I believe, and saw Gray, someone I had met on a handful of occasions, sitting towards the back of the vehicle, staring with a fixed expression into vacancy. It was a trance not to be disturbed by a fellow passenger. Perhaps even then he was brooding, prophetically upon this, the latest and most exceptional of his many publications. Gray will be familiar to many as a regular contributor to magazines, newspapers and think tank pamphlets, in which domains he certainly cannot be accused of rigidity. At times, he has sounded like a green, a euro-enthusiast, a new social democrat and much else.

Officially, he is Professor of European Thought at the LSE, where it is difficult to avoid the marks of confinement among neo-Darwinians. His publisher summarises his intellectual journey in this way: "a former supporter of the New Right, he has since revised his views, and now believes that the conventional political solutions of conservatism and social democracy are no longer viable."

This book demonstrates, however, that it is not only the political habits of mind of the recent centuries which Gray disputes. The purpose of Straw Dogs is to insist that humanism, religion, science, moral philosophy and eco-politics are all dead. Homo rapiens is, like the dinosaurs, doomed to early extinction. No wonder he's lost patience with the Third Way, Guardian editorials and reform of the Mental Health Act.

Gray's argument is that Plato, Christianity and post-Enlightenment humanism have fallen into the same fundamental error of believing that human beings are different from other animals and therefore, in some way, not subject to the decisive laws of Darwinian evolution, which dictate that the purpose of life is physical procreation.

For Gray, humanism's belief in any other type of progress "is only a secular version of Christian faith." He approvingly quotes James Lovelock, the philosopher of Gaia, that "the human species is now so numerous as to constitute a serious planetary malady." The human "plague animal" is clever enough only to ensure that he is equipped with technologies guaranteed to make the process of self-destruction more rapid than might otherwise be the case.

Those who think that it may not yet be too late to resist this Malthusian catastrophe are fiercely dismissed. The Greens's belief in better planetary management is another deluded humanism. Those neo-Darwinians who insist that, in spite of evolution, human beings enjoy meaningful choices between good and evil, true and false, are rebuked for denying the purity of Darwinian truth: that human talent is wholly at the service of evolutionary success, not truth or virtue. Only Gaia, Gray says, correctly judges that "human life has no more meaning than the life of slime mould."

Gray's aim, one imagines, is to do for humanism ("a secular religion thrown together from decaying scraps of Christian myth") what Richard Dawkins' genetic fundamentalism did for religion. In so far as the book has anything to tell us about how to live, conduct our politics or relate to each other, it recommends the primitive, proto-religious impulse of animism which Gray, without too much evidence, supposes might be the way it is for animals and perhaps even slime mould. His book concludes: "Other animals do not need a purpose in life. A contradiction to itself, the human animal cannot do without one. Can we not think of the aim of life as being simply to see?"

It is easy to dismiss a book so obviously calculated to scandalise anyone blessed with the gift of literacy, but to do so is to misunderstand the purpose of the writer of prophecy or tract, which is to induce action.

At one level, Straw Dogs is the idlers' gospel, a tract for J.G. Ballard's workless society of "a billion balconies facing the sun," but Gray is honest enough to admit that in reality, those who have any choice in the matter are working more, not less.

Written more as a series of "pensees" than as an essay with an integrated argument, its manner is somewhere between Nostradamus, Pascal and writings of the Unabomber. Because it is more of an illustrated act of insistence than an integrated piece of reasoning it can, like, say, the book of Ecclesiastes, be made to appear consistent with more or less any contemporary development. Genetic manipulation: man over-reaches himself. Re-emergence of Islamic fundamentalism: nothing ever really gets better. Vanity of vanities, all is vanity.

Gray's anatomy of the human condition, however, is not the political and moral cul-de-sac it purports to be. Let us indulge in an act of faith and assume that he is right: humanity's days are numbered. Somewhere out in eternity lie apocalypse and oblivion.

Does this mean we should cease striving for greater social equality, more environmentally careful technologies, effective birth control and peace in the Middle East? Hardly. Gray's fire and brimstone may or may not make us shudder, but after it, we must return to our daily tasks, our minds and souls more alert to the chances which lie before us. That is the goal of most religious and moral writing and Straw Dogs, whether Gray likes it or not, has a great deal in common with religious prose: it is ambitious, universal and based more upon faith and instinct than reason.

Perhaps the man on the Cambridge omnibus is heading for a life of considered and elevated passivity, destined for the fenland mysticism of St Julian of Norwich. But I doubt it. Gray is too much the preacher to stop worrying about what we're all up to. Nor can I help wondering how Straw Dogs will sell among the slime mould.

Ian Hargreaves is professor of journalism at Cardiff University and former editor of the Independent

Straw Dogs is available from Amazon (UK).