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Animals learn by trial and error, and the smarter they are, the fewer trials they need. Traveling backward buys us many trials for the price of one, but traveling forward allows us to dispense with trials entirely. Just as pilots practice flying in flight simulators, the rest of us practice living in life simulators, and our ability to simulate future courses of action and preview their consequences enables us to learn from mistakes without making them. We don't need to bake a liver cupcake to find out that it is a stunningly bad idea; simply imagining it is punishment enough. The same is true for insulting the boss and misplacing the children. We may not heed the warnings that prospection provides, but at least we aren't surprised when we wake up with a hangover or when our waists and our inseams swap sizes. The dark network allows us to visit the future, but not just any future. When we contemplate futures that don't include us--Will the NASDAQ be up next week? Will Hillary run in 2008?--the dark network is quiet. Only when we move ourselves through time does it come alive.
Perhaps the most startling fact about the dark network isn't what it does but how often it does it. Neuroscientists refer to it as the brain's default mode, which is to say that we spend more of our time away from the present than in it. People typically overestimate how often they are in the moment because they rarely take notice when they take leave. It is only when the environment demands our attention--a dog barks, a child cries, a telephone rings--that our mental time machines switch themselves off and deposit us with a bump in the here and now. We stay just long enough to take a message and then we slip off again to the land of Elsewhen, our dark networks awash in light.
Gilbert and Buckner are professors of psychology at Harvard. Gilbert's Stumbling on Happiness was published last May.