Madness: A Brief History by Roy Porter (Oxford, £7.99)

This, his eighty-somethingth (tallies vary), is Roy Porter's last book. He wrote on numerous subjects - the 18th century, gout, medicine - but madness was the topic he kept returning to. It is certainly one he knows his way around, where the fruitful stories lurk. Are we meant to catch an echo of Porter's productivity in the case, described here, of Clifford Beers, who in 1903 dictated his asylum autobiography (80,000 words long) in 90 hours?

The title of Beers's work, A Mind that Found Itself, is not an example of deranged wishful thinking. It was a cry of protest from his own personal experience, was read by William James, and served as a blueprint for his organisation, the National Committee for Mental Hygiene. It marked one of the rare times when the testimony of the insane was taken seriously. Which seems as good a point as any to remind you that Porter's (by all accounts electrifying) plenary address to the first conference of the European Association of the History of Psychiatry was titled "Listening to Insanity".

That is the very subtly addressed agenda behind this book, it would appear. The delusions of the insane can stretch our definitions of the word. Porter tells the story of a Parisian tailor during the reign of terror who became unhinged with fear when he challenged the execution of Louis XVI and then misconstrued an overheard conversation. All perfectly understandable under the circumstances, but he ended up being incarcerated in an asylum. Only the enlightened approach of a Dr Philippe Pinel saved him: he dressed up three of his colleagues as magistrates representing the revolutionary legislature and had them pronounce the tailor's patriotism beyond reproach, which resulted in an immediate end to his symptoms.

This is a short book - brief enough, as Dr Oliver Sacks remarks, to fit in the pocket - but a dense one. In this we hope it is denser than the Diagnostic and Statistical Manual of the American Psychiatric Association , which Porter drily notes "requires energetic revision every few years". The second edition was 134 pages long; the latest, published in 2000, runs to 943. The APA voted to remove homosexuality from its list of psycho-pathologies in 1975.

So, clearly another title for this book could have been "a history of unsatisfactory diagnoses and treatments in the field of mental illness". You can incarcerate in the hell-holes of Bedlam, or you can claim that insanity is merely a normative construct of the establishment, as Laing and Foucault did. You can believe that the conditions are physical or mental in origin; responses to internal or external pressures; yet the problem of classification and treatment persists.

It is as much a problem of definition as anything else. Porter is a lucid pragmatist, laying all the salient facts and interpretations out before us in as unbiased a manner as possible, although he has time enough for Richard Hunter and Ida Macalpine's 60s overview of the matter to quote it at some length: "Aetiology remains speculative, pathogenesis largely obscure, classifications predominantly symptomatic and hence arbitrary and possibly ephemeral; physical treatments are empirical and subject to fashion, and psycho-therapies still only in their infancy and doctrinaire." This is not going quite as far as to say, like Thomas Szasz, that psychiatry is a "pseudoscience" - but it's not far short of it.

This is a distillation of many productive years' thinking about the subject, and should you want a short, readable, and possibly unarguable overview of the subject, here it is. As a frighteningly high proportion of us will suffer some kind of mental illness in the course of our lives, you may find it a good idea to keep this to hand. Just in case.

· Buy Madness at Amazon.co.uk