Seligman is a cognitive psychologist who asked the question: what makes a person pick themselves up after rejection by a lover, or another keep going when their life's work comes to nothing? He found that the ability of some people to bounce back from apparent defeat is not, as we sentimentally like to say, a 'triumph of the human will'. Rather than having an inborn trait of greatness, such people have developed a way of explaining events that does not see defeat as permanent or affecting their basic value. Nor is this trait something that 'we either have or we don't'; optimism involves a set of skills which can be learned.

Pessimistic people tend to think that misfortune is their fault. The cause of their specific misfortune or general misery is, they believe, permanent - stupidity, lack of talent, ugliness - therefore they do not bother to change it. Few of us are wholly pessimistic, but most of us will have given pessimism free reign in reaction to particular past events. In psychology textbooks, such reactions are considered 'normal'. But Seligman says it does not have to be this way, that a different way of explaining setbacks to yourself ('explanatory style') will protect you from letting crises cast you into depression. If you have even an average level of pessimism, Seligman says, it will drag down your success in every arena of life: work, relationships, health.

At the exact same point that a pessimist will wilt, an optimist perseveres and breaks through an invisible barrier. Not getting through this barrier is often misinterpreted as laziness or lack of talent; Seligman found that people who give up easily never dispute their own interpretation of failure or disparagement. Those who regularly 'vault the wall' listen to their internal dialogue and argue against their own limiting thoughts, quickly finding positive reasons for rejection.

For a book about optimism, it is slightly ironic that Learned Optimism draws much of its data from studies of depression. Seligman asks: why is there so much depression around these days? He argues that our historically recent preoccupation with individualism creates its own form of mental shackle. Invited to believe in our own endless possibilities, any form of failure becomes devastating. Combine this with the crumbling of previously solid psychological supports - the Nation, God, the Family - and we have an epidemic of depression. However, where drugs like Prozac can be effective in eliminating it, there is a gap between successfully treated depression, and habitual optimism. With the positive explanatory style that Seligman recommends, problems are seen as temporary, specific and external, rather than inevitable expressions of our failure as a person. Cognitive therapy changes the basic way a person sees the world, and that altered perception tends to be permanent.

Learned Optimism is an important work within the self-help field because it provides a scientific foundation for many of the claims made in it. It is a bestseller because it attracted the readers who normally would consider personal development ideas as, to use the author's phrase 'metaphysical boosterism'. The book is therefore not simply about optimism (although it may well turn you into an optimist) but the validity of personal change itself and the dynamic nature of the human condition. In that respect it will remain relevant well into the new century.