If it is true that the piano man was deliberately faking symptoms of mental illness, up to and including being found in the sea by the police while supposedly attempting suicide, then we must not be too hard on the unfortunate professionals who were taken in by him. For one thing, he had apparently worked with the mentally ill, so he may have known what symptoms the doctors were seeking. For another, faking it is not hard for someone who is determined to pull it off.

In a famous Californian experiment during the early 1970s, eight perfectly sane volunteers managed to spend several weeks in a variety of mental hospitals impersonating schizophrenics, without being detected. When questioned afterwards, all the staff who treated them were completely convinced that the fakers had the illness. Indeed, in some cases, it took quite an effort by the research organisers to persuade the authorities to release them.

Sustaining the symptoms of that illness (delusions, hallucinations, crazy utterances) over such a long period would be much harder than piano man's pretence of speechlessness. Such elective muteness and faux-autism merely requires not saying anything and avoiding much eye contact. Keeping up the pretence that you think you are Jesus Christ or John Lennon for weeks on end is another matter. Since anyone making such claims would be given extremely powerful anti-psychotic drugs, you would also have to put up with the treatment's horrid side effects.

It would be even harder to sustain the role of a seriously depressed person for months on end if you were happy and content. Sooner or later you would be bound to cheer when your football team scored or let slip that "it's a beautiful day" when the sun came out. Hardest of all would be the non-verbal signals. The gloomy features on your face and the down-at-heel hunched posture would give way to the odd cheeky smile or jaunty best-foot-forward on your way to lunch.

But all that is assuming you are normal and only faking it as an experiment or for a jape. It is likely that piano man is, in fact, suffering from quite serious identity problems, probably borderline personality disorder.

Sufferers have a weak sense of self and the adoption of multiple personae is common. Indeed, they often have sub-personalities. Of course, all of us have different personae, for example, when at work or at home, but we know about them and they are nowhere near as extreme. With sub-personalities, you do not know about the other people you are.

When reality is unbearable, a good way to evade it is to be someone else, and at least three-quarters of people with sub-personalities have suffered severe maltreatment as children. If you are being sexually abused, for example, one way to escape the emotional discomfort of the situation is to pretend to be someone else for its duration.

If piano man developed the sub-personality of a mental patient and based it on prior knowledge of psychiatric criteria, that would have made him undetectable because, when in the role, he actually would have been that elective mute, person-avoidant, apparently very anxious person.

Most likely, it was unplanned. When the police came upon him in the surf, piano man may not have been sure whether he was intending to kill himself, and, instead of adopting a corporeal escape from a troubled mind, he decided to be someone else. Instead of dying or actually becoming crazy, he sought refuge in a sub-personality. Ironically, therefore, he may have remained sane only by pretending to be mad.

The whole incident is just one more of the many proofs that there is not, and may never be, a simple test for who is sane, not even a physical one. It's unlikely ever to be possible to define who is mad through genetic tests or scanning brainwave patterns and brain electro-chemistry because the line dividing sanity and insanity is just too fine.

While the certifiable might be found to be generally different in these measures, it's highly probable that a good proportion of politicians, business leaders and pop stars would have the same biological profile.

Spare a thought, then, for the professionals who cared for the piano man. In all probability he was seriously disturbed and their care and attention enabled him to return to being the man with the name on his birth certificate.

· Oliver James is a clinical psychologist and the author of They Fuck You Up: How to survive family life, published by Bloomsbury