By Molly McCloskey

One of the many interesting questions to emerge from Andrew Solomon's grand tour of depression is why the condition ever survived evolution's cuts. It doesn't promote reproduction; severe depression doesn't promote much of anything at all (except perhaps insight, empathy, and depth of character, but more on that later). Further, while the depressed appear to have a more accurate view of themselves and the world than do the non-depressed, such clarity has not been a selective advantage. Solomon quotes the author of a book called Positive Illusions: 'Normal human thought and perception is marked not by accuracy but positive self-enhancing illusions about the self, the world, and the future...' The explanation Solomon favours is that while depression itself may serve little purpose, the emotional range of which it is a part is valuable enough to justify its own extremes. Viewed in the context of a continuum - rather than as a condition of 'categorical purity' - depression is something we eradicate at our peril.

According to statistics quoted here, 28 million Americans - that's one in 10 - are now on selective serotonin reuptake inhibitors (SSRIs), of which Prozac is one. Why? Solomon is at pains to show that depression is not a recent invention of the middle classes, but rather that we are living in a time in which we have a vocabulary for such distress and are allowed to use it - without, for instance, being accused of demonic possession - but that we are also living in a time of unprecedented fragmentation, choice and pressure.

The Noonday Demon explores our long, long walk with 'the black dog' (as Samuel Johnson liked to call it), taking us back to the fifth century B.C., when Hippocrates imagined a physical cure for the melancholy he viewed as a mixture of internal and environmental factors, recognised the existence of what we now term SAD, and also employed the talking cure. Solomon brings us to Cambodia, to Greenland, to the legislative chambers, state hospitals, and research centres of the US, and into the minds and lives of sufferers. Into these stories he interweaves his own seven-year negotiation with the illness, including a breakdown he suffered while writing the book.

But the questions he keeps coming back to are age-old: what is depression and what should we do about it? Depression, he says succinctly, 'is an emotional pain that forces itself on us against our will, and then breaks free of its externals.' As for the slow magic worked by drugs: 'You can feel it happening, how the medication seems to be poisoning the parasite so that bit by bit it withers away.'

Since Prozac was launched in 1987, there's been something of a backlash against anti-depressants, mostly centred around their overprescription and how this may ultimately affect our capacity, not to mention our willingness, to withstand a range of emotions.

'New medications that are being developed may likely make it quick, easy, cheap, and safe to block many unwanted emotions,' says one researcher. 'We should be there within the next decade.'

If so, who will we be? What is the relationship between the depressed self, the 'normal' self, and the medicated and functioning self? ('I am so tired,' one woman says, '...of trying to figure out who I am when I'm 'fine'...') Solomon disagrees with the notion that there is an uncorrupted core self that medication masks and, in his own experience, 'all the medication in the world can provide no more than a way for you to reinvent yourself. The medicine will not reinvent you.'

Abstaining from medication may have even more dire consequences. Multiple depressive episodes can actually ravage neuronal tissue, altering the structure and biochemistry of the brain. And then there's the life-cost. Solomon tells of a women who, having decided her depression was stress-related, one by one eliminated all the causes of stress in her life, which turned out to be the very things most of us consider essential: a job, a partner, a social life. But, she tells him, with a rather downbeat pride, 'I did it without pills.' He compares her response to the illness with his own (he's been on medication and in therapy for seven years) and is frankly appalled by her choices.

'Prozac should not obviate insight; it should enable insight,' says one researcher. Not only drugs, Solomon argues, gut depression itself grants insight. Quite simply, it's a sign that something isn't working. 'You need to run your life differently from how you ran it before.' The horrifying loneliness of depression teaches the value of intimacy. Our strengths are our weaknesses; if we can love, we can also experience a sometimes unbearable sense of loss. Solomon returns again and again to the integral role love plays in recovery, as well as to the input sufferers themselves have. 'Anti-depressants help those who help themselves.' Have a sense of humour. Don't wish your life away. Neither blame nor indulge yourself.

He veers throughout between poetic evocations of the darkness and practical tips for surviving it. In embracing depression's hard lessons - defending its right to exist, really - and in delving into its cultural, historical and medical milieux, he has produced an admirable and important work, both beautifully written and unashamedly useful.