Phil Baty talks to Roy Porter (right), the historian of medicine whose knowledge of medieval plagues was used by a government confronting the horrors of Aids

Roy Porter does not care for academia. Growing-up in a "completely un-intellectual and un-academic" working class home in South London's New Cross Gate, he learned to read, he says, "as if by magic", confounding his mother by picking up books at "a very early age indeed". Bitten by the bug of reading, he became a historian of medicine and psychiatry "by some grotesque accident". And he seems to have been apologising for his academic credentials ever since.

Professor of the social history of medicine at the Wellcome Institute for the History of Medicine, Porter is happy in splendid isolation - at the Euston Road ivory tower, which is funded by the profits of the Glaxo-Wellcome drug empire. "I'm fortunate that I have escaped the burden of academic administration," he says. "And here we didn't suffer the drastic cuts Margaret Thatcher chose to inflict upon the profession out of spleen in the 1980s."

A large man, with wild, receding hair and a thatch of facial stubble that creeps down his throat to meet the wayward chest hairs spilling from his half-buttoned shirt, he is adamant about his instinctive allegiance to the non-academic community. "I've never been into academic politics, I don't care for academic gossip and I have no academic ambitions," he says, pushing up the sleeves of his battered denim jacket to reveal a chunky silver bracelet.

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"I see academics hanging on into their sixties, seventies or eighties, and somehow feeling that their entire identity is tied up with being professor emeritus of this or that. That's not my scene."

Porter read history at Christ's College in Cambridge in the 1960s. A contemporary of the irreverent historian Simon Schama, he became a research student at Christ's, then a research fellow, before moving to Churchill College in 1972 as director of studies in history. He joined the Wellcome Institute in 1980 and has been there ever since.

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"I've been wonderfully privileged," he says, "and I'm trying to pay back my debt to society." So privileged, indeed, that Porter's work has been overwhelmingly populist. With a sideline in journalism, and as a rent-a-quote social commentator, Porter is a celebrity historian, writing on everything from the social history of London to the creation of sexual knowledge 1650-1950. But it is as a historian of medicine and of psychiatry in particular that he is probably best known.

So what about the long-running and fraught debate within psychiatry about whether people are born mad or become so because of their environment? Contemporary psychiatry belongs largely to the biologists and neurologists. The future is thought to lie with new drugs to control the chemical imbalances thought to be responsible for madness or, even, perhaps with genetic engineering. Psychotherapy, the talking cure designed to put right years of insults from parents, siblings, lovers, seems less and less useful.

Porter has studied the history of madness - monitoring claims and counter claims in the intellectual battles between the church, the physicians and later the psychiatrists. From the Greeks, who would let blood or purge bile from the bodies of the mad to redress supposed chemical imbalances and restore the "humours", to those who will today pop Prozac in a less crude attempt to alter the chemical make-up of the mind, Porter has been observing.

So can psychiatrists learn anything from the lessons of history? Porter sits on the fence on that one. "It seems to me that living in the culture and language that we do, we cannot help but think of ourselves as minds and bodies. And we cannot help but think of the relationship between them as mysterious. Anybody who wants to dissolve that mystery with a kind of reductionism which says that consciousness is all just chemical fails to take into account the very fact of the identity and personality that we cart along with us. Everyone has cultural baggage from several thousand years and personal baggage from 20 years or whatever. But unless you believe in disembodied souls and things then you work on the assumption that consciousness is in some sense constrained by bodily existence."

The phenomenon of Prozac, he says, is a classic case where the lessons of the past have been pushed aside as the media and medics scramble to hail the green and white pill as the wonder drug. "Prozac is so big these days. I went to America recently and it surprised me. I kept taking people to bars and they'd say they couldn't drink becasue they were on Prozac. I don't like the punitive mythology which says that everything has its cost and its price. But I still wait to be shown a drug that doesn't have any costs or side-effects. If you look back at all the histories of drugs and sedatives and intoxicants, narcotics, and stimulants, there's always some cost that has to be paid, even though at the time people would say 'this is the wonder drug'. You can go back to the 18th century and people said the 'great thing about opium is that it's non-addictive, it has no harmful side-effects, it is God's gift to the universe' and it never turned out to be like that. If Prozac turns out to have no side- effects and no harmful consequences, then it's the first such drug that they've ever developed."

The thrice-married Porter is unsure about recent reports that Prozac may affect the libido. "I only got as far in America as offering people a drink rather than a screw," he laughs.

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High spirits are perhaps the key to Porter's disregard for the often persuasive picture of the history of psychiatry painted by the late French philosopher Michel Foucault. Foucault plotted the fate of the insane as a snowballing descent from a medieval golden age, when the mad enjoyed a unique freedom and dialogue with the sane, into a disaster of ostracism, confinement and oppression. For Foucault, each successive era of psychiatry was progressively worse. When the bondage of 18th-century Bedlam was belittled in 1796 by the Tukes, a small Quaker family in York, who discarded the chains and manacles and inititated a new, and highly successful "moral therapy", Foucault dismissed the development. A move from iron manacles to emotional, "mind forg'd manacles", was not progress, he insisted.

It is not hard to challenge the details of the Foucaultian analysis, but in Porter's book, Mind Forg'd Manacles: Madness and Psychiatry in England from Restoration to Regency, Foucault's basic premise and approach is dismissed. "I'm very hostile at gut level to the Foucaultian approach," says Porter. "I have no faith in golden age myths. It seems to me that the past was in all sorts of ways nasty and violent and bigoted. I find Foucault's bleakness rather frightening. He appears to be unable to discriminate between relatively liberal, humane and tolerant regimes and relatively authoritarian and dictatorial regimes. For Foucault there doesn't seem to be much difference between, say, Hitler and a democracy - because everything gets reduced to some form of power.

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"If, like Foucault, you pronounce that mankind died in the sands at the beginning of the 20th century, then it seems to me that what you are doing is to deny the possibility of knowledge being anything other than a form of power which governs and thwarts and controls rather than knowledge or consciousness being a form of creativity."

Porter's blend of history is envigorated by colourful and vivid anecdotes. He has always tried to marry his technical medical expertise, he says, with social and cultural history and a passion for literature, enjoying the "interplay between psychiatry as specialist knowledge and the public understanding of so-called crazy people".

It was in 1985, when the Aids virus came to the fore, that Porter first found himself involuntarily thrust into the limelight. "Suddenly the public got interested in the complex ambiguities of modern disease and modern medicine," he explains. "The history of medicine moved from being a sleepy backwater to being headline news. I can remember in 1985/6/7 being endlessly rung up by policy makers at the Department of Health and Social Security - as well as by the media. They wanted to know how people dealt with epidemics in the past when there was cholera, or plague or whatever. I got involved in it not just because it was history, but because it was life and death."

Does he really believe his research can inform modern policymaking? "I know there's a school of professional historians who say it's a grave blunder to look at the past with the eyes of the present and the present through the eyes of the past, because you just end up with Whigism," he says. "But that's too precious. It really mattered that we got what we did about Aids right.

"I wrote an article for the British Medical Journal at the end of 1986 - 'History says no to policeman's response to Aids'. This is the time when there was a loud voice from the tabloids and Tory backbenchers in favour of forcible quarantining or tattooing or whatever. But the historians found parallells from the past."

Indeed, Porter finds an ideal illustration in the treatment of venereal disease in the early 20th century. A penal approach was developed in Scandinavia and Germany, he says. People were legally required to be tested, were compelled to give details of partners and those partners were "hunted down". "It just didn't work," says Porter. "People avoided doctors like the plague. At the same time Britain pioneered a voluntary, anonymous approach. It made more sense in terms of civil liberties, and it made more medical sense.

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"I have some faith in the power of collective memory," he elaborates. "I do think that consciousness expands and I wouldn't do history if I didn't think that. I think that doing history is a way of accessing more and more of consciousness and that gives you a wider sense of difference and diversity and hopefully of tolerance. If I didn't believe that, then it would seem perverse to carry on with the humanities."