Looking for the upside of depression, The New York Times Magazine approaches the "disease" from the point of view of evolutionary biology. "For Darwin, depression was a clarifying force, focusing the mind on its most essential problems. In his autobiography, he speculated on the purpose of such misery; his evolutionary theory was shadowed by his own life story. 'Pain or suffering of any kind,' he wrote, 'if long continued, causes depression and lessens the power of action, yet it is well adapted to make a creature guard itself against any great or sudden evil.' And so sorrow was explained away, because pleasure was not enough. Sometimes, Darwin wrote, it is the sadness that informs as it 'leads an animal to pursue that course of action which is most beneficial.' The darkness was a kind of light. The mystery of depression is not that it exists — the mind, like the flesh, is prone to malfunction. Instead, the paradox of depression has long been its prevalence. While most mental illnesses are extremely rare — schizophrenia, for example, is seen in less than 1 percent of the population — depression is everywhere, as inescapable as the common cold."