“Many of us are familiar with mind-wandering in a number of guises: procrastination, reflection, meditation, self-flagellation, daydreaming. But while some mental meandering seems fruitful, on other occasions it has the unmistakeable bite of a bad habit, something that holds us back from reaching our full potential. Reverie can be a reprieve from reality and a font of inspiration, yes. But equally familiar is the mind’s tendency to devolve into sour and fruitless rumination when left to its own devices, especially when we’re in the grip of depression, anxiety or obsession.”