Last week, irked by what I saw as the use of wild exaggeration by church leaders in the embryology Bill debate, I challenged one of them - the Bishop of Durham - to justify one of his more outrageous claims. Tom Wright had accused the “militantly atheist and secularist lobby” behind the Bill (a Bill, as it happens, supported and sponsored by many practising Christians) of believing “that we have the right to kill unborn children and surplus old people.”

I didn't choose to quarrel with Dr Wright's characterisation of abortion. What I did ask for, however, was any evidence whatsoever that any significant secular or atheist body of opinion advocates “the right to kill surplus old people”.

Bishop Wright's reply to my challenge, carried on Thursday's letters page in The Times, was to refuse to reply to it until I had answered a further series of questions that he set for me. This is, of course, odd. A cynic might think that the Bishop was playing for time while a diocesan search squad parsed the texts of old Polly Toynbee columns looking for gerontocide.

So let me answer the Bishop's questions. “Is there,” he asks me, “any difference between humans and other animals and does this difference matter?” Yes, humans are the most advanced species on earth in terms of emotional and psychological capacity and yes, that makes a difference. But it doesn't, for me, affect whether research can be carried out on hybrid cell clusters.

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BACKGROUND

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• Brown compromise over embryo Bill revolt

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Question two. What makes me think I “can reduce the function of religion to the provision of ‘comfort and companionship'” instead of seeing it as a “public truth”? Being an atheist, I suppose. I see religion as a cultural and psychological construct, which fulfils certain almost universal needs and which, as a consequence, I am disinclined to condemn. I am more than content to live alongside the religious in amity, but I don't think I should be expected to acknowledge a public truth that I actually think is a public myth.

The Bishop's third question, “where in St Paul's letters to the Corinthians - or anywhere else for that matter - does the Apostle attack the ‘sinful mixing' which Mr Aaronovitch seems to think is the sole subject matter of Leviticus?” is based on a misunderstanding of what I wrote, which I daresay is my fault. The point I was attempting to make is that the religious change their own rules when it suits them. What was an abomination in the Old Testament was suddenly permitted under the Apostles, and particularly by Paul. If this wasn't the case the Bishop would be (and here I speculate) a whole foreskin lighter.

So Bishop, your turn. Who is it, of any significance whatsoever, who advocates the “killing of surplus old people”?

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