…the thought process that defines the disorder, which is known as rumination. (The verb is derived from the Latin word for “chewed over,” which describes the act of digestion in cattle, in which they swallow, regurgitate and then rechew their food.) In recent decades, psychiatry has come to see rumination as a dangerous mental habit, because it leads people to fixate on their flaws and problems, thus extending their negative moods. … There is nothing profound about depressive rumination. There is just a recursive loop of woe.

The bleakness of this thought process helps explain why, according to the Yale psychologist Susan Nolen-Hoeksema, people with “ruminative tendencies” are more likely to become depressed. They’re also more likely to become unnerved by stressful events… And then there are the cognitive deficits. Because rumination hijacks the stream of consciousness — we become exquisitely attentive to our pain — numerous studies have found that depressed subjects struggle to think about anything else… The end result is poor performance on tests for memory and executive function, especially when the task involves lots of information. (These deficits disappear when test subjects are first distracted from their depression and thus better able to focus on the exercise.) Such research has reinforced the view that rumination is a useless kind of pessimism, a perfect waste of mental energy.

If depression didn’t exist — if we didn’t react to stress and trauma with endless ruminations — then we would be less likely to solve our predicaments. Wisdom isn’t cheap, and we pay for it with pain.