When the sea of faith recedes, what remains?

If you dispense with a deity in your life and thought, is there nothing more than a God-shaped hole in your existence? Familiar questions probably describe the gulf between believers and unbelievers better than any number of neat arguments over the worth of science or the value of philosophical proof.

It feels, as often as not, like a dispute over what is and is not imaginable for the human animal. People ask about life's purpose and meaning. Many cannot conceive of an answer that does not include some version of God. The atheist says we can live well, be fulfilled and complete, without such a prop. Or as Peter Watson would have it, the need to live in something - the world and all it contains - should trump the need to live for something.

It's a hot topic in this 21st of the Christian centuries, but why that should be remains unexplained. Because faith has become weak and the vanity of secularism too strong? Or because superstition (as the atheist would say) still holds too tight a grip when science shows that we should know - that we do know - better?

If truth be told, the reasons for arguments over faith are more interesting than the arguments themselves. How can God be harmed by a lack of belief? What odds does it make to the atheist if millions cling to miracles? It might be a sad day for mass education, but scientific truth is not eradicated, or so a stern unbeliever might conclude, by nonsense. Despite it all, each side holds the other to be bad news for the species. To paraphrase Watson, how we seek to live, collectively, is of consuming importance.

But how many sides are there, exactly, in this running battle? Watson is honest enough, in this fine intellectual history of atheism, to admit that the godless have often enough included disreputable types you wouldn't necessarily want on your team. Faith meanwhile comes in all shapes and sizes, in degrees of intensity and sincerity. There is more to this tale than the usual clerics versus a few stentorian authors stumbling over the idea of tolerance.

Plenty of sincere believers disown plenty of preachers, dishonest or sincere. Some atheists (this reviewer, for one) hold the claims of science to be beside the point. Down the years any amount of nonsense has been held up as "scientific truth". Phlogiston theory was hot (so to speak) science for the best part of a century, but it wouldn't aid Richard Dawkins today. Hiding behind scientific method, with its conjectures and refutations, doesn't alter the fact that trust in science is often betrayed.

Hence Watson's subtitle and his purpose. The Age of Nothing is a book that takes the notion of the God-shaped void seriously and tries to find its answers in how we could - or should - live. The author looks at fundamentals such as happiness, poetry, "the fuller life" found "not from one portmanteau idea, as religions typically offer ... but from a collection of altogether smaller ideas achieved piecemeal". The experience of art is therefore central, a fact Dawkins attempted to address in 1998's Unweaving the Rainbow with its arguments for "poetic science", its conceit that Keats and Newton, alert to one another, "might hear the galaxies sing".

Yet religion, by no accident, has also taken poetry and all arts seriously for a very long time. Dawkins concedes, implicitly and explicitly, what the churches have known since their origins: art is the attempted expression of what it means to exist. Without it, "the facts", of faith or of science, fail to nourish and satisfy. Nevertheless, the failures are equivalent.

In some important ways, as Watson's book shows, those unadorned facts fail to convince. God, real or dead since Nietzsche's obituary in 1882, involves statements over what it means to exist, but for the poor human beast existence, brief enough, is the business at hand. The Age of Nothing has a great deal to say about living well by paying attention in an era in which, as Thomas Mann wrote: "the idea of one overbearing truth is exhausted". Since Watson is unshakeable in his atheism, God remains "the greatest and most overbearing idea there is". But so what?

The author is keen on phenomenology, less as a coherent school of philosophy than as a regime for living. Watson would have us lower our eyes from God and smell the coffee, instant by instant, stitching together a self from the discontinuous narrative of experience. He meanwhile wants us to bring existence back to a human scale, without a deity or metaphysics. A dizzying host of artists and philosophers are summoned to his side.

Lurking within a big, satisfying book is the idea of consolation as it was once understood by religious writers. Watson is not the kind of atheist who can be content with the nothing in his title. Instead of commandments he finds morality in evolutionary imperatives causing us - despite some evidence to the contrary - to co-operate. He wants us to engage with the natural world. He wants us to grasp that a "consequential" life has less to do with posterity than with self-respect, dignity and "coherence", this of itself "being a form of beauty".

The Age of Nothing will insulate some atheists against doubt (and irony too, perhaps). It ought to give the godly plenty to think about and the agnostic fruitful lines of inquiry. It has the virtue of refusing to concede everything to scientific rationalism. But the book feels, for all that, like a shoring up of defences. At one point Watson admits that the authentically secular are probably those too busy getting on with life to give much of a toss about a failure to believe. He is not among them.

"Happy-fulfilled"; the "performance of living"; living "in the moment": there is an air of self-help manual (and of gospel) in some of this, no matter how distinguished the names in the citations. It tends towards that paradox, a secular theology, even when Watson is unflinching in his intellectual allegiances. The Age of Nothing sticks to the conviction that even amidst the great nothing we need, must need, something.

The philosophers of religion could have a lot of fun with that, and why not? It amounts to a kind of deviationist atheism. If there is a glory in non-belief it is, surely, that nothing - the worth of our time on the planet least of all - need be justified. But Watson, like so many others, must have his contest with superstition. He could as well say that if there is no God existence is futile, pointless and cruel. If there is a God, only the word "comical" need be added. It would serve.

This is a book offering great value, nevertheless, to anyone who ever imagined that a Dawkins or a Christopher Hitchens provided the first, last, or most original word on what it means to deny ancient fictions. Watson has the best witnesses - and most of the best tunes.