The answer, Gilbert says in his book Stumbling on Happiness, is time travel.
The human mind can move through time from past to present or present or past to future and in any gear. Our ability to imagine future events is a recent evolutionary development, and unparalleled in the animal kingdom. "We are a race of time travelers," Gilbert says "unfettered by chronology and capable of visiting the future or revisiting the past whenever we wish. If our neural time machines are damaged by illness, age or accident, we may become trapped in the present. Alzheimer’s disease, for instance, specifically attacks the dark network, stranding many of its victims in an endless now, unable to remember their yesterdays or envision their tomorrows."
Neuroscientists refer to the dark network it as the brain’s default mode, which is to say that we spend more of our time in other modes. Posted by Casey Kazan.