Hanna Segal opens the door at the second ring. She turned 90 two days previously and is small, creased and walnut-brown, with a crop of startling white hair and a pair of eyes that look suitably gimlet-like but are, she observes dispassionately, not much use to anyone these days.

"Like the rest of me, really," she adds.

"I haven't been analysed myself for a couple of years now, but I would love some help in coping with a body that's started to behave as if it were a psychotic infant. It's like some narcissistic baby. Nothing is good enough for it. Nothing is ever quite right."

Segal is one of the most eminent psychoanalysts ever to have practised in Britain. Splendidly, despite living in this country for nearly 70 years, she speaks English with a great, gutteral mittel-European accent. Even better, while she has been forced to abandon her beloved pipe, every few minutes she takes a small wad of tobacco from a tin at her elbow, pops it surreptitiously into her mouth, and chews. Who does that, these days? It is impossible not to warm to her.

Hers is rather a strange profession, though. All that poking around in the dark, walled-off corners of people's minds, hunting down explanations for bizarre adult behaviour in obscure childhood events that invariably involve breasts or toilets. A lot of people have no time for it.

Segal, obviously, does. "The more I think about it," she says, "the importance lies in seeking truth. Not 'The Truth' with a capital T, an omniscience, but truth that is the same as reality. All we are really looking for, in a patient on the couch, is a distinction between lies and truth."

She no longer has a couch in her study, although the room, on the ground floor of large house in north London, is suitably sombre and book-lined. She stopped seeing individual patients a couple of years ago, although she still supervises major cases, from all over the world, by telephone.

But the kind of people who came to see her were, she says, generally those who "seek to avoid truth, and so end up in delusion. What you are aiming to achieve is a change in the direction of the mind, a bend towards truth. And while all science aims at truth, psychoanalysis is unique in recognising that the search for truth is, in itself, therapeutic."

The latter phrase is one penned by Segal and colleagues for the obituary of Melanie Klein, of whose work Segal is pretty much universally recognised as the most prominent postwar interpreter. Kleinian psychoanalysis is one of the two main schools within the British Psychoanalytical Society, the other being Freudian (after Anna, Sigmund's daughter).

The nuances between the two are not immediately apparent to the layperson, but if you ask Segal why, many years ago, she opted for the former rather than the latter, she answers that Freud's masterwork, The Ego and the Mechanisms of Defence, read "like a textbook. It didn't speak to my imagination at all." Klein's Psychoanalysis of Children, on the other hand, "was a revelation. It opened up a whole new world."

Segal read both books on the advice of the distinguished Edinburgh psychoanalyst Ronald Fairbairn, just after the outbreak of the second world war, in Edinburgh. She had fled there from Paris, where her parents were then living and where she had grudgingly decided to complete the third year of her medical studies. The Nazi invasion of Poland in 1939 had made it impossible to finish her degree at the University of Warsaw.

"I wanted to get back, desperately, with some of my friends, to help fight the invader. But I couldn't; there were just no more trains. So more by luck than reason, really, I survived. If ever I had managed to get back I certainly wouldn't be here today."

Segal was born into a well-to-do Jewish family in August 1918 in Lodz: "Poland's equivalent of Manchester. A horrible, industrial place. Ugh." Her father was a successful lawyer, although when she was 12 or 13 the family moved with him to Geneva, where he had taken a job editing one of the publications of the League of Nations. The international school she found herself in was an eye-opener. "It did me a power of good. I'd been brought up deeply patriotic, but there I learned that Poland was not the centre of the universe. Thanks to that school, I have no particular religious tradition that alters or moulds my view of the world. All that was knocked out of me. It was immensely formative, terribly important."

It was in Geneva that Segal first read the works of Sigmund Freud. "I read Proust first, before Freud," she says. "And I think I simply realised that there was nothing, absolutely nothing, more fascinating than human nature. And human relations."

So when the time came to choose a career, psychoanalysis was almost a natural choice. It satisfied her interest in human nature, assuaged a powerful social conscience ("I have to feel I am doing something useful. Something that might help people"), and allowed her to explore the third great passion in her life: art. Segal's major contribution to the world of psychoanalysis is most probably in aesthetics and what is known as symbolisation. Two of her best-known books are entitled Dream, Phantasy and Art, and Delusion and Artistic Creativity.

"We cope with our anxieties and desires," she explains, "in symbolical ways. We all need a capacity for symbol formation, or symbolisation: hopefully, we will try to find someone like our mother to marry, rather than try actually to marry our mother. Artists exist on the borderline of severe psychotic anxieties: if they succeed in symbolising them, then they can produce great art - but if not, they can be in trouble."

Segal indicates a painting on the wall of a rather chaotic vase of flowers, all violent purples, magentas and reds. "Look closer," she says. "Can you see the face of a dog?" It's there, sort of; a gloomy bloodhound, all drooping jowls and sad, tortured eyes. "The work of a patient," she says. "Saved him from madness."

I must look dubious, because she comes back at me with Van Gogh, whose story you really can't argue with. She cites another case that, in many ways, first opened her to the possibilities of psychoanalysis: many years ago, on an evacuation train before the war, a girl had a schizophrenic fit, screaming what sounded like hysterical nonsense.

"She kept shouting, 'I shat my lover in the loo! I shat my lover in the loo!'" Segal says. "Later, after I read Klein, I realised that girl's words actually had a very obvious meaning; you could understand them. She was the one being evacuated, but in her mind she had reversed that situation: she was the one doing the evacuating. This, I understood, was the language of subconscious fantasy."

I'm still dubious. How can psychoanalysts ever really know they're right? "We can't," she says. "There's no quick cure or absolute certainty. And the truth rarely stays the truth; yesterday's truth is not today's. But there is a sense of accumulating evidence. You mustn't concentrate, or try to remember - but in the mass of patient communication, you have to select the right fact, with an open mind. And you have to be sure that fact is not just your idea, your own, overrated idea. It's not easy."

Segal has been president of the British Psychoanalytical Society, Freud Professor of Psychoanalysis at the University of London, and vice-president of the International Psychoanalytical Association. For a long time she steered clear of sociopolitical issues, until in 1983 she co-founded a pressure group called Psychoanalysts for the Prevention of Nuclear War. She is now outspoken about a range of issues, including 9/11 and the 2003 invasion of Iraq. "We're always told not to treat society as if it were a patient on the couch," she says, "but group psychology can be understood because it is a group of humans."

The function of a group, she contends, is certainly to work together, but also to act as a kind of repository for our projections of all those bad things we cannot tolerate in ourselves. "Groups contain our psychotic anxieties and delusions," she says (and has argued, in a quite remarkable paper entitled September 11).

"Generally, we delegate what you might call the 'mad' functions - fighting, religion - to subgroups: the army, the church. But those subgroups must be under the control of the working part of the group. My point is that when mad things start happening, it's when subgroups get out of control, and particularly when they combine: God, money and the military is a particularly deadly recipe." The Iraq conflict, she argues, was about "the need for an enemy", and "a religious fanaticism linked to, and covering up, mass robbery".

Today, Segal believes, our collective sanity is threatened by "a delusional inner world of omnipotence, and absolute evil, and sainthood. Unfortunately, we also have to contend with mammon." And since we tend to submit to the tyranny of our own groups, "speaking our minds takes courage, because groups do not like outspoken dissenters." The battle now "is between insanity based on mutual projections, and sanity based on truth". And all we, as citizens, can do is "struggle to expose lies, and strive for the preservation of sane human values".

She is not convinced she will ever see that battle resolved ("This is my last interview," she says gaily. "You'd better do a good job.") But the important thing, she insists, adopting the vivid symbol she first found in Cormac McCarthy's post-apocalyptic fable The Road, is to "keep a little fire burning; however small, however hidden. I find this extraordinarily helpful: we live in a mad world, but for those of us who believe in some human values, it is terribly important that we just keep this little fire burning. It is about trusting your judgement, and the power of love. A little trust, and a little care".

· Yesterday, Today and Tomorrow, a collection of writings by Hanna Segal, is published by Routledge (£21.99).