ANY LIST of great ideas of the 20th century, or indeed any century, is bound to be individual. The idea of powered flight permitting the exploration and closing of the globe, the ideas of quantum physics, the theory of the electron within matter (permitting solid state electronics, including, among numerous other things, e-mail and the Internet), the idea of rocket propulsion, which has permitted the exploration of outer space - all these may be cited as important, even central, ideas of the 20th century.

But the idea of psychoanalysis occupies a special place in the history of ideas of the 20th century because it does not belong to specialists. The vocabulary of psychoanalysis has become our working vocabulary of emotional life. Terms such as "ego", "ambivalent", "unconscious", "neurotic", "resistance" and "repression" are words we routinely use to describe emotions and motivations.

Psychoanalysis is the emotional language of the 20th century. It is the language of the novel and, implicitly or explicitly, the emotional language of film - whether in fun, as with Woody Allen, as serious analysis, as in Robert Redford's film of a family tragedy, Ordinary People, or as a mixture of both, as in the forthcoming psychoanalytic gangster parody, Analyse This, with Billy Crystal's psychiatrist tracing the troubles of Robert De Niro's gangster through the ins and outs of each man's relationship with his father.

Psychoanalysis permits each of us to become the poet of our own experience. By means of the psychoanalytic dialogue, we can discover our feelings and the often complex and confusing mental states that we all experience at times, and express them for ourselves. Psychoanalysis has helped millions of people to love and to work. But it is not in the spirit of psychoanalysis to be self-congratulatory.

Psychoanalysis offers a humane alternative treatment to the too often desperate medical attempts to treat human mental pain by surgical, physical and chemical intervention. But psychoanalysis is not a panacea, a miracle cure, as in the fantasised magic bullets of modern psychopharmacology. Psychoanalysis, with its emphasis on exploring tension, ambiguity, ambivalence, conflict and contradiction, cannot offer itself as any other than tentative and limited. As Freud put it, psychoanalysis can only offer the possibility of transforming neurotic misery into common unhappiness.

The idea of psychoanalysis is that the human inner world can be understood. In its prolonged listening sessions, extending over the same amount of time that it takes to learn a new language, psychoanalysis, through Freud's great discovery of the "analytic hour", offers an entry point into exploring and making sense of the inner world of human experience. Seen in this way psychoanalysis becomes, and in fact is, an extension of Western science into an area that was previously reserved for the artist.

Freud, who trained in one of the elite laboratories of Europe, above all wanted to understand. What was the meaning of dreams, of strange, compulsive bedtime rituals, of paralysis of the limbs of the body with no organic cause; the meaning of the individual who will speak only in a foreign tongue?

Freud was the pioneer theorist of psychoanalysis and, as with all intellectual pioneers, we can still admire and respect his approach - in his case, to understanding the human psyche. Like Newton, who was wrong in every essential, Freud, as the pioneer, created a framework of understanding, showing that there existed a way to create effective useful understandings of the previously incomprehensible phenomena of the inner world of human beings.

So what are the psychoanalytic analogies, if any, with Galileo's discovery of the moons of Jupiter, the discovery that shook the medieval world view by showing that there could be other centres of motion than Earth? Indeed there is just such an discovery, one that shakes our socially produced assumptions about the world to their foundations.

Just as the medieval world view had Earth as the centre of the natural world, the modern Western view has made the individual the centre of the social world. We inhabit a culture that has made a fetish of individual action, at the expense of an understanding of the complex processes by which the individual is made and behaves in interaction with culture.

Whereas Freud, coupled strongly to the individualistic assumptions of the 19th century, theorised that the central conflict of human mental life was connected to drives to reduce biological tensions arising from inside the individual, particularly aggression and sexual tension, the Scottish psychoanalyst Ronald Fairbairn was the first to observe that psychoanalysis in fact showed that there existed a fundamental human drive towards relationships and connections with others.

We now know how necessary relationships are for human mental well-being, not just in infancy, but throughout life.

Our attachments both allow us to thrive and define who we are. Contemporary psychoanalysis now offers a fundamental challenge to our ideas about ourselves.

From the perspective of a modern psychoanalytic relational point of view we can see how much distress is caused by our culture's denial of the importance of human relationships and attachments. The work of maintaining the emotional connections through which we thrive has fallen to women, while many men have attempted to live a life of a mythological male independence, a destructive and dysfunctional social ideal.

From being an expert whom people consult about emotional difficulties, where diagnosis and a preordained progress of treatment would then follow, the analyst now is more usefully seen as an experienced guide to the exploration of a terrain that is uniquely the individual's own. With its combination of clinical evidence and its theory of human emotional needs, psychoanalysis offers the next century a potentially invaluable entry point for increasing our understanding of the ubiquity of human aggression that has so disfigured the 20th century.

The writer is a psychoanalytic psychotherapist. His book `Cassandra's Daughter: A History of Psychoanalysis in Europe and America' is to be published by Penguin on 26 August