Some years ago I wrote a short sketch for a late night revue at the York Festival which asked the audience to spare a thought for the daft. The sketch began by outlining the various kinds of daftness from which people typically suffered: pretty daft, plain daft, and, the most extreme category, brush daft. It then went on to make a plea for understanding and sympathy. "After all, at this very moment there might be a completely daft person sitting right next you." Of course, the joke depended on everyone's easygoing assumption that it was most unlikely there was anyone daft sitting right next to them. That's why I doubt if it would get many laughs today. Today, we realise only too well that even those who share many of our views on art, literature, and politics are still only too capable of having a thoroughly daft side.

Consider Marina. For years she has been a successful radio producer. I've taken part in one or two of her documentaries and developed the highest regard for her perception, intelligence and sense of humour. But there we were in the lift at Broadcasting House the other day and I found myself telling her about my sister's recent serious illness. 'It's not looking too good,' I told her as the lift sped toward the ground floor. 'She's getting weaker and weaker and we still don't have a convincing explanation.'

'Do you have a photograph of her,' Marina said. For a second I assumed that she'd like to see what my poor sister looked like so that she could more readily empathise with my sadness about her predicament. Such sensitivity. 'No, I'm so sorry. I don't have one on me.'

But it turns out that the photograph was wanted for an entirely different reason. As the lift bounced to a halt at the ground floor, Marina, thoughtful, clever, rational, informed Marina said: 'Because if I had a photograph I could try and perform a Reiki aura analysis.' 'A what?' 'An aura analysis. It doesn't always work but sometimes it shows up the place in the body where the aura is absent. It can help in diagnosis.'

I was so shocked by this sudden evidence of Marina's newfound daftness that I couldn't manufacture a rational response. 'Oh,' I said, 'I'm afraid that I only have one photograph of her at home and I wouldn't want to risk losing that.' Marina looked downcast. 'I could get you a copy, though.' 'A copy's no use,' she said, as though I had failed to grasp a Euclidian axiom. 'It must be the original photograph.'

Even as the lift doors opened I thought about confronting her with the news that a photograph was in itself already a copy and therefore presumably disqualified from her analysis. But I decided instead to concentrate on all those other aspects of Marina that I'd so liked in the past. I smiled warmly. 'Well, see you around,' I said, and made for the revolving door.

If it weren't for Sartre I'd have no serious problems about this encounter. I could allow myself to believe that my conversation with Marina revealed nothing more than a small part of her entire character. She'd just gone a bit daft. She'd handed over a very small corner of her otherwise rational brain to daftness.

But this is precisely what Sartre ridiculed as the 'mosaic' theory of character, the notion that it is possible for a person to hold some totally irrational opinions (he was talking specifically about fascistic views) and yet still be a loving son and a good spouse and a solid, reliable, intelligent member of the community. 'Nonsense,' said Jean-Paul. If we hold fascistic views then these attitudes will inform every other aspect of our character. They can't be treated like pieces of coloured mosaic that simply lie alongside each other without touching the wider picture. The conclusion is rather shocking. I can no longer regard Marina as a little bit off her chump. I have to face up to the fact that she may well now be wholly daft.

Does this mean that I should now desert her (along with all those other friends who display any form of commitment to the new irrationalisms of our age) or should I be honest and confront her with my diagnosis of her condition? That could well create a further problem. What do I do if she insists on aura evidence?