What’s the difference between a guru and a crackpot? Adam Lively shows how to tell them apart. The Queen in Alice Through the Looking-Glass took pride in believing six impossible things before breakfast. In fact, there is probably not a human being alive who doesn’t hold some wholly irrational belief. As Anthony Storr laconically points out, everybody secretly believes themselves to be more important and significant than they in fact are. Some have claimed to be Messiah or God.

But if we are all deluded, how are we to judge when an individual has tipped over the edge from run-of-the-mill delusion into insanity? As soon as we start looking at madness from the point of view of the beliefs people hold, it seems that we enter a looking-glass world in which the sane are madder than we think, and perhaps the mad saner.

It is this shadowy world that Storr enters in his book on charismatic spiritual leaders. His examples of such leaders range from those one might easily suspect of mental illness - such as David Koresh and Jim Jones, who led the mass suicide at Jonestown, Guyana in 1978 - to those, such as Jesus, who have established enduring world religions. He also has chapters on Freud and Jung, neither of whom would have considered themselves gurus.

Storr’s great strengths as a writer are the sensitivity with which he applies his experience as a practising psychiatrist to cultural phenomena, and his ability to synthesise elegantly, with an apt use of quotation, what previous thinkers have written on a subject. Music and the Mind, his previous book, may not have contained any startling new insights, but it was an eloquent and heart-felt plea for the importance of music to the human psyche. Feet of Clay, similarly, does not present any grand theory, but does illustrate richly some characteristics that gurus seem to share.

One of the best of biographical chapters is that on Saint Ignatius of Loyola, founder of the Jesuits, who provides a memorable example of the way someone’s becoming a guru is often preceded by a psychological or spiritual crisis, an episode of severe depression or some similarly intense and absorbing mental disturbance that leads to a temporary withdrawal from the world and reality.

Jesus had his 40 days in the wilderness, Jung has a mid-life crisis that in Storr’s judgment amounted to a psychotic episode, and Ignatius spent three months sitting on a rock overlooking the river Cardoner. In the case of the latter, the transformation wrought by withdrawal was particularly dramatic. Before, he was a dueller, a ladies’ man and a gambler. After, he was a fervent advocate and practictioner of extreme asceticism and spiritual discipline.

But, as Storr acutely observes: “The aggression of the soldier has not disappeared; it has become redirected against a new enemy, the sinful former self.” Ignatius’ aggression and iron strength of character in achieving this inner transformation was what attracted followers to him.

In other cases, the guru’s need for disciples to shore up his ego - and the techniques of emotional manipulation used to achieve that end - are more in evidence. Many gurus, Storr points out, have had isolated childhoods, screened from the need to negotiate and compromise with the emotional needs of others on the basis of equality. In adult life the guru remains isolated, with followers rather than friends. Relationships are an all-or-nothing affair, a choice between submission and rejection. In extreme cases, such as those of Koresh and Jones, this need to assert power over others may extend to sadism and sexual abuse.

Storr’s most important conclusion is that it is in these areas of the guru’s relationship with others that we should look for evidence of insanity, rather than in the supposed irrationality of the guru’s beliefs. On the whole, he avoids this pitfall himself - even when his scientific scepticism is most on show, as in his somewhat cursory dismissal of Jung’s fascinating thoughts on the occult and synchronicity.

The trouble with drawing a distinction between behaviour and belief is that if one believes with enough conviction, and if the belief demands it, then just about any behaviour can seem rational. If Feet of Clay lacks the sharp-ness of focus to cut through this philosophical Gordian knot, it does provide a fascinating overview of the subject.