I've always enjoyed Thought for the Day (TFTD), that two-minute spot in the middle of Radio 4's Today programme, which seems to be a brief respite from the hard news, and a chance for someone to give moral or ethical reflections on current events. The trouble is that only religious speakers are invited. Rabbis, priests, imams, chaplains, and monks are there, but never humanists, agnostics, or atheists.

Why not? Wouldn't it be better if they were? Morality is not the sole prerogative of the religious – there are even reasons to think that the irreligious are more moral. So why shouldn't we be invited to speak on TFTD?

This week Today will, according to Gavin Orland, get about 1,500 letters of complaint. He has set up a scheme in which people have pledged to write to the BBC and demand that TFTD either be abolished or opened up to non-believers.

I signed his pledge because I think the faithless have lots to contribute, but the issues are not as simple as some have suggested.

I have contributed to the equivalent spots on local radio – to TFTD on Radio Bristol and, now that I have moved, to Pause for Thought on Radio Devon. The two were very different.

Radio Bristol recorded live and the editor demanded that it be topical – not just vaguely topical but right up to the minute. The brief said, "Try to use the news agenda of the day as your theme". This meant preparing the night before, and ringing him for approval, which often he did not give, and then rewriting it ready to cycle off in the early morning to the studio. His objections were confusing; "No bad-mouthing religions" sounds fine, until you discover that you cannot say something like "we don't need God to make us act morally" or "psychology explains why we can be moral creatures without being religious". OK, I managed to rewrite my pieces avoiding such things, but it was tricky. And it seemed so unfair, when the vicars and rabbis could go on about God's will, heaven and hell, and what Christ wants for his people. Needless to say, this editor was a Christian.

When doing my last Bristol TFTD, I learned from the studio manager that others had found it difficult too, and Radio Bristol now had only one other atheist left to call on, happily the bright and articulate Julian Baggini.

Devon is, so far, quite different. The editor is very relaxed, the show is pre-recorded and the brief is to say something relevant, but not necessarily based on today's news. I have only done six so far, but am hopeful that this will work out better.

These troubles raise the basic question of what TFTD is for. At Bristol we once had a day-long meeting with the contributors and editor to discuss TFTD. And did we argue! The written brief says "religions have some deep and challenging things to say about the way we live our lives" and "express these eternal truths in the modern vernacular". It goes on … don't be "homespun or folksy", "Don't be safe or trite. It is called 'Thought for the Day' for a reason" – but I am not sure that any of us was ever clear what that reason is.

And here is the challenge for Radio 4. Yes, please do keep TFTD. I don't want to see it axed. Yes, please do invite we atheists, humanists and brights to contribute, but first let's work out just what TFTD is for. If it's to bring a moral bearing to current events, then let the atheists talk about evolved altruism, how to live well without believing in God or fearing eternal retribution, and how to make decisions about abortion, death, suicide, love or pain without religious guidance. If it is to be inspiring, then let unbelievers use the eternal questions of philosophy without any assumed eternal answers.

When I hear the words "Thought for the day" I imagine someone giving me a thought to take me through the troubles of a typical day; a thought that will help me see myself, or others, or the world around me more constructively or more truly. Others may have other ideas, but I hope the editors at Today will take this challenge seriously, work out what TFTD is for, and then decide whether we non-believers have something worthwhile to contribute. I think they'll find we have.