When the lady in the Bibliothèque nationale had cast doubt on the psychological state of the letter-writer, and questioned that I should even attempt such a search without getting a psychiatric report, I had initially dismissed her. But if the letter-writer really was mad, should I not drop my search immediately?

I contacted a clinical psychologist at St Thomas’s Hospital in London who reassured me. ‘I see no evidence of mental illness,’ she said after I had recounted my story, but then added, less helpfully, ‘Unless of course she’s made the whole thing up and there is no Maurice at all, in which case she’s obviously completely mad.’

But she agreed that this was unlikely. ‘Traditionally, in many societies, one year of mourning is seen as the time needed to recover from loss. In the case of the loss of a child, it is far longer. This woman seems to have no support from other people within her family, just the one friend. By writing this letter, sending this bottle, she is marking the end of her formal bereavement. And she is getting rid of a great deal of self-directed anger. Casting her story to the waves provides a form of rebirth for her.’

So not mad, or rather, not more mad than such a traumatic event would justify, just terribly sad.

I asked a friend who I knew had lost a son fifty years earlier – half a century before – how long it had taken her to be able to talk of his death. ‘Until now,’ she replied in a low voice. ‘I have never spoken of it until now.’ And then she told me how her three-year-old son had died and how life had not continued in any meaningful way for many years thereafter.

I also described the letter-writer’s state of mind as well as I could to a general medical doctor. Having had my confidence in my analysis of Maurice’s death so badly shaken by the realization that he may not have drowned, I now wanted as many people as possible with different areas of expertise to provide their opinions, just in case they picked up on some clue that had completely escaped me, or that I had wrongly interpreted.

To be fair, the doctor had very few actual symptoms on which to base his diagnosis. After I had assured him that in the circumstances I would settle for general impressions rather than a precise diagnosis, he agreed to share his opinion.

Perhaps trauma, he hazarded. A head injury, prolonged unconsciousness. A road accident, maybe. Soon he warmed up. A coma, perhaps, hence ‘between two lights’. Unlikely to be cancer, since he left ‘without warning’ but probably not sudden, since he ‘slipped away’.

Perhaps meningo-encephalitis. A meningococcal meningitis. There has been a lot of it around in recent years. That could lead to cerebral changes, coma. Maybe a cerebral tumour. Intake of ecstasy can cause cerebral effects. That would perhaps also tie in with her anger and over-indulgence of Maurice.