By Richard Bernstein

About a third of the way through his all-encompassing, brave and deeply humane book, "The Noonday Demon: An Atlas of Depression," Andrew Solomon writes a vivid description of something called an ndeup (pronounced nn-DUPP). An ndeup is an animist ritual performed by the Lebou and Sérèr peoples of Senegal to appease the spirits that in the local view cause depression, the disease that had become the main and preoccupying substance of Mr. Solomon's life. He traveled to West Africa to understand the varieties of depression and he underwent the ritual, allowing himself to be smeared from head to toe with ram's and chicken's blood while African women danced and stomped around him to the driving beat of drums.

His ndeup is fascinating in itself, and culturally instructive; he concludes that it was probably better than many forms of group therapy for depression practiced in the United States. But his description of the ndeup also reflects the remarkable scope of Mr. Solomon's book: its depth, its breadth and its vitality.

"The Noonday Demon" is one of those rare volumes that deserve the adjective "definitive." It combines powerfully felt personal experience with a tremendous sense of intellectual discovery. It is open-minded, critically informed and poetic at the same time, and despite the nature of its subject it is written with far too much élan and elegance ever to become depressing itself.

"Shortly before my 31st birthday, I went to pieces," Mr. Solomon writes. He readily acknowledges that he might seem to have been an unlikely victim of what used to be called melancholia, especially melancholia of a severe variety, one in which Mr. Solomon wept until the skin around his eyes became chapped. He was wealthy and well connected, and he was achieving success as a journalist and novelist.

But his happy circumstances are precisely what invoke the first lesson that Mr. Solomon teaches about depression: it is like any other disease, cancer or pneumonia. It strikes independently of the objective condition of one's being, and when it does strike, you don't just snap out of it.

In one intense passage he uses the metaphor of a vine slowly choking a vast oak to bring home the insidious power of mental illness:

"In the end I was compacted and fetal, depleted by this thing that was crushing me without holding me. Its tendrils threatened to pulverize my mind and my courage and my stomach, and crack my bones and desiccate my body. It went on glutting itself on me when there seemed nothing left to feed it."

Mr. Solomon travels not only to West Africa but also to Greenland and Cambodia in an effort to appreciate the universality of depression's grip. He gets to know a large contingent of fellow sufferers, whose stories he tells. He reads everything, from Hippocrates to Freud to Michel Foucault to lay out for us the variety of opinion and philosophy that the subject of depression has occasioned.

He discusses why the disease occurs twice as frequently in women and what its relationship to violence may be in men. He delves into the possible Darwinian meanings of depression, wondering what satanic kink in the process of evolution could have allowed it in the first place.

He fathoms the deeper meanings of his own affliction, finding within his struggle with sadness and loss something redemptive, a tragic sense that brings with it pain but also insight and beauty. He reviews the debates over various therapies, most important between psychotherapy and psychobiology — that is between talking to a therapist and taking drugs — and he finds that the debate has to do with the connection of biochemistry and personal responsibility.

"The conflict between psycho-dynamic therapy and medication is ultimately a conflict on moral grounds," he writes. "We tend categorically to assume that if the problem is responsive to psychotherapeutic dialogue, it is a problem you should be able to overcome with simple rigor, while a problem responsive to the ingestion of chemicals is not your fault and requires no rigor of you. It is true both that very little depression is entirely the fault of the sufferer, and that almost all depression can be ameliorated with rigor. Antidepressants help those who help themselves."

Mr. Solomon also plunges intrepidly into his personal history: the death of his mother at 58, his struggles with unpopularity in grade school, his discovery of his homosexuality and the implications of that discovery for his psychological condition. He does not spare himself in this book. He tells about having unprotected, anonymous sex with men as part of a plan to contract AIDS, a form of suicide that would, in his mind, have absolved him of moral responsibility for his death. Luckily, he did not contract H.I.V. He talks at one point about becoming violent himself, savagely beating a former lover who, he believed, had cruelly betrayed him, and he writes with rigorous honesty about his feelings afterward:

"Engaging in violent acts is not a good way to beat depression. It is, however, effective. To deny the inbred curative power of violence would be a terrible mistake. I came home that night covered with blood — mine and his — and with a feeling of both horror and exhilaration. I felt tremendous release."

Among the most moving passages here are those that describe the suicide of his mother, who, suffering from terminal ovarian cancer, gathered her husband and two sons around her and passed away in their presence. "Don't think you're paying me some kind of great tribute if you let my death become the great event of your life," she told Mr. Solomon. That is a magnificent injunction to issue to a troubled son, but it was not an injunction that Mr. Solomon was able to obey, and he provides a nuanced and subtle account of why that was so.

One of the many virtues of Mr. Solomon's work — perhaps its most important virtue — is its fearlessness. Having intimate knowledge of the noonday demon, he is open-minded and full of compassion, but he is also not afraid to pass judgments: on unworkable concepts, on bad therapy, on the habit of self-delusion.

His book is fabulously rich, informed by science, literature, moral philosophy and above all by his own determination to see his subject whole, not to avoid any topic, any idea, any conclusion, as long as it brings him and us closer to the truth.