The doctrine of purgatory offers a preposterous revelation of the way the theological mind works. Purgatory is a sort of divine Ellis Island, a Hadean waiting room where dead souls go if their sins aren't bad enough to send them to hell, but they still need a bit of remedial checking out and purifying before they can be admitted to the sin-free-zone of heaven. In medieval times, the Church used to sell 'indulgences' for money. This amounted to paying for some number of days' remission from purgatory, and the Church literally (and with breathtaking presumption) issued signed certificates specifying the number of days off that had been purchased. The Roman Catholic Church is an institution for whose gains the phrase 'ill-gotten' might have been specially invented. And of all its money-making rip-offs, the selling of indulgences must surely rank among the greatest con tricks in history, the medieval equivalent of the Nigerian Internet scam but far more successful.

As recently as 1903, Pope Pius X was still able to tabulate the number of days' remission from purgatory that each rank in the hierarchy was entitled to grant: cardinals two hundred days, archbishops a hundred days, bishops a mere fifty days. By his time, however, indulgences were no longer sold directly for money. Even in the Middle Ages, money was not the only currency in which you could buy parole from purgatory. You could pay in prayers too, either your own before death or the prayers of others on your behalf, after your death. And money could buy prayers. If you were rich, you could lay down provision for your soul in perpetuity. My own Oxford College, New College, was founded in 1379 (it was new then) by one of that century's great philanthropists, William of Wykeham, Bishop of Winchester. A medieval bishop could become the Bill Gates of the age, controlling the equivalent of the information highway (to God), and amassing huge riches. His diocese was exceptionally large, and Wykeham used his wealth and influence to found two great educational establishments, one in Winchester and one in Oxford. Education was important to Wykeham, but, in the words of the official New College history, published in 1979 to mark the sixth centenary, the fundamental purpose of the college was 'as a great chantry to make intercession for the repose of his soul. He provided for the service of the chapel by ten chaplains, three clerks and sixteen choristers, and he ordered that they alone were to be retained if the college's income failed.' Wykeham left New College in the hands of the Fellowship, a self-electing body which has been continuously in existence like a single organism for more than six hundred years. Presumably he trusted us to continue to pray for his soul through the centuries.

Today the college has only one chaplain*) and no clerks, and the steady century-by-century torrent of prayers for Wykeham in purgatory has dwindled to a trickle of two prayers per year. The choristers alone go from strength to strength and their music is, indeed, magical. Even I feel a twinge of guilt, as a member of that Fellowship, for a trust betrayed. In the understanding of his own time, Wykeham was doing the equivalent of a rich man today making a large down payment to a cryogenics company which guarantees to freeze your body and keep it insulated from earthquakes, civil disorder, nuclear war and other hazards, until some future time when medical science has learned how to unfreeze it and cure whatever disease it was dying of. Are we later Fellows of New College reneging on a contract with our Founder? If so, we are in good company. Hundreds of medieval benefactors died trusting that their heirs, well paid to do so, would pray for them in purgatory. I can't help wondering what proportion of Europe's medieval treasures of art and architecture started out as down payments on eternity, in trusts now betrayed.

But what really fascinates me about the doctrine of purgatory is the evidence that theologians have advanced for it: evidence so spectacularly weak that it renders even more comical the airy confidence with which it is asserted. The entry on purgatory in the Catholic Encyclopedia has a section called 'proofs'. The essential evidence for the existence of purgatory is this. If the dead simply went to heaven or hell on the basis of their sins while on Earth, there would be no point in praying for them. 'For why pray for the dead, if there be no belief in the power of prayer to afford solace to those who as yet are excluded from the sight of God.' And we do pray for the dead, don't we? Therefore purgatory must exist, otherwise our prayers would be pointless! Q.E.D. This seriously is an example of what passes for reasoning in the theological mind.

That remarkable non sequitur is mirrored, on a larger scale, in another common deployment of the Argument from Consolation. There must be a God, the argument goes, because, if there were not, life would be empty, pointless, futile, a desert of meaninglessness and insignificance. How can it be necessary to point out that the logic falls at the first fence? Maybe life is empty. Maybe our prayers for the dead really are pointless. To presume the opposite is to presume the truth of the very conclusion we seek to prove. The alleged syllogism is transparently circular. Life without your wife may very well be intolerable, barren and empty, but this unfortunately doesn't stop her being dead. There is something infantile in the presumption that somebody else (parents in the case of children, God in the case of adults) has a responsibility to give your life meaning and point. It is all of a piece with the infantilism of those who, the moment they twist their ankle, look around for someone to sue. Somebody else must be responsible for my well-being, and somebody else must be to blame if I am hurt. Is it a similar infantilism that really lies behind the 'need' for a God? Are we back to Binker again?

The truly adult view, by contrast, is that our life is as meaningful, as full and as wonderful as we choose to make it. And we can make it very wonderful indeed. If science gives consolation of a non-material kind, it merges into my final topic, inspiration.

This is a matter of taste or private judgement, which has the slightly unfortunate effect that the method of argument I must employ is rhetoric rather than logic. I've done it before, and so have many others including, to name only recent examples, Carl Sagan in Pale Blue Dot, E. O. Wilson in Biophilia, Michael Shermer in The Soul of Science and Paul Kurtz in Affirmations. In Unweaving the Rainbow I tried to convey how lucky we are to be alive, given that the vast majority of people who could potentially be thrown up by the combinatorial lottery of DNA will in fact never be born. For those of us lucky enough to be here, I pictured the relative brevity of life by imagining a laser-thin spotlight creeping along a gigantic ruler of time. Everything before or after the spotlight is shrouded in the darkness of the dead past, or the darkness of the unknown future. We are staggeringly lucky to find ourselves in the spotlight. However brief our time in the sun, if we waste a second of it, or complain that it is dull or barren or (like a child) boring, couldn't this be seen as a callous insult to those unborn trillions who will never even be offered life in the first place? As many atheists have said better than me, the knowledge that we have only one life should make it all the more precious. The atheist view is correspondingly life-affirming and life-enhancing, while at the same time never being tainted with self-delusion, wishful thinking, or the whingeing self-pity of those who feel that life owes them something. Emily Dickinson said,