The institute is rejecting this system because it does not lead to useful research. It is starting afresh, with a focus on how the brain and its trillions of synaptic connections work. The British Psychological Society rejects the centrality of diagnosis for seemingly quite different reasons — among them, because defining people by a devastating label may not help them.

Both approaches recognize that mental illnesses are complex individual responses — less like hypothyroidism, in which you fall ill because your body does not secrete enough thyroid hormone, and more like metabolic syndrome, in which a collection of unrelated risk factors (high blood pressure, body fat around the waist) increases your chance of heart disease.

The implications are that social experience plays a significant role in who becomes mentally ill, when they fall ill and how their illness unfolds. We should view illness as caused not only by brain deficits but also by abuse, deprivation and inequality, which alter the way brains behave. Illness thus requires social interventions, not just pharmacological ones.

ONE outcome of this rethinking could be that talk therapy will regain some of the importance it lost when the new diagnostic system was young. And we know how to do talk therapy. That doesn’t rule out medication: while there may be problems with the long-term use of antipsychotics, many people find them useful when their symptoms are severe.

The rethinking comes at a time of disconcerting awareness that mental health problems are far more pervasive than we might have imagined. The World Health Organization estimates that one in four people will have an episode of mental illness in their lifetime. Mental and behavioral problems are the biggest single cause of disability on the planet. But in low- and middle-income countries, about four of five of those disabled by the illnesses do not receive treatment for them.

When the United Nations sets its new Sustainable Development Goals this spring, it should include mental illness, along with diseases like AIDS and malaria, as scourges to be combated. There is much we still do not know about mental illness, and much we can do to improve its care. But we know enough to do something, and to accept that knowing more and doing more should be a fundamental commitment.