What would you do right now if you learned that you were going to die in ten minutes? Would you race upstairs and light that Marlboro you've been hiding in your sock drawer since the Ford administration? Would you waltz into your boss's office and present him with a detailed description of his personal defects? Would you drive out to that steakhouse near the new mall and order a T-bone, medium rare, with an extra side of the really bad cholesterol? Hard to say, of course, but of all the things you might do in your final ten minutes, it's a pretty safe bet that few of them are things you actually did today.

Now, some people will bemoan this fact, wag their fingers in your direction, and tell you sternly that you should live every minute of your life as though it were your last, which only goes to show that some people would spend their final ten minutes giving other people dumb advice. The things we do when we expect our lives to continue are naturally and properly different than the things we might do if we expected them to end abruptly. We go easy on the lard and tobacco, smile dutifully at yet another of our supervisor's witless jokes, read books like this one when we could be wearing paper hats and eating pistachio macaroons in the bathtub, and we do each of these things in the charitable service of the people we will soon become. We treat our future selves as though they were our children, spending most of the hours of most of our days constructing tomorrows that we hope will make them happy. Rather than indulging in whatever strikes our momentary fancy, we take responsibility for the welfare of our future selves, squirreling away portions of our paychecks each month so they can enjoy their retirements on a putting green, jogging and flossing with some regularity so they can avoid coronaries and gum grafts, enduring dirty diapers and mind-numbing repetitions of The Cat in the Hat so that someday they will have fatcheeked grandchildren to bounce on their laps. Even plunking down a dollar at the convenience store is an act of charity intended to ensure that the person we are about to become will enjoy the Twinkie we are paying for now. In fact, just about any time we want something—a promotion, a marriage, an automobile, a cheeseburger—we are expecting that if we get it, then the person who has our fingerprints a second, minute, day, or decade from now will enjoy the world they inherit from us, honoring our sacrifices as they reap the harvest of our shrewd investment decisions and dietary forbearance.

Yeah, yeah. Don't hold your breath. Like the fruits of our loins, our temporal progeny are often thankless. We toil and sweat to give them just what we think they will like, and they quit their jobs, grow their hair, move to or from San Francisco, and wonder how we could ever have been stupid enough to think they'd like that. We fail to achieve the accolades and rewards that we consider crucial to their well-being, and they end up thanking God that things didn't work out according to our shortsighted, misguided plan. Even that person who takes a bite of the Twinkie we purchased a few minutes earlier may make a sour face and accuse us of having bought the wrong snack. No one likes to be criticized, of course, but if the things we successfully strive for do not make our future selves happy, or if the things we unsuccessfully avoid do, then it seems reasonable (if somewhat ungracious) for them to cast a disparaging glance backward and wonder what the hell we were thinking. They may recognize our good intentions and begrudgingly acknowledge that we did the best we could, but they will inevitably whine to their therapists about how our best just wasn't good enough for them.

How can this happen? Shouldn’t we know the tastes, preferences, needs, and desires of the people we will be next year—or at least later this afternoon? Shouldn't we understand our future selves well enough to shape their lives—to find careers and lovers whom they will cherish, to buy slipcovers for the sofa that they will treasure for years to come? So why do they end up with attics and lives that are full of stuff that we considered indispensable and that they consider painful, embarrassing, or useless? Why do they criticize our choice of romantic partners, second-guess our strategies for professional advancement, and pay good money to remove the tattoos that we paid good money to get? Why do they experience regret and relief when they think about us, rather than pride and appreciation? We might understand all this if we had neglected them, ignored them, mistreated them in some fundamental way—but damn it, we gave them the best years of our lives! How can they be disappointed when we accomplish our coveted goals, and why are they so damned giddy when they end up in precisely the spot that we worked so hard to steer them clear of? Is there something wrong with them?

Or is there something wrong with us?

When I was ten years old, the most magical object in my house was a book on optical illusions. Its pages introduced me to the Müller-Lyer lines whose arrow-tipped ends made them appear as though they were different lengths even though a ruler showed them to be identical, the Necker cube that appeared to have an open side one moment and then an open top the next, the drawing of a chalice that suddenly became a pair of silhouetted faces before flickering back into a chalice again (see figure 1). I would sit on the floor in my father's study and stare at that book for hours, mesmerized by the fact that these simple drawings could force my brain to believe things that it knew with utter certainty to be wrong. This is when I learned that mistakes are interesting and began planning a life that contained several of them. But an optical illusion is not interesting simply because it causes everyone to make a mistake; rather, it is interesting because it causes everyone to make the same mistake. If I saw a chalice, you saw Elvis, and a friend of ours saw a paper carton of moo goo gai pan, then the object we were looking at would be a very fine inkblot but a lousy optical illusion. What is so compelling about optical illusions is that everyone sees the chalice first, the faces next, and then—flicker flicker—there's that chalice again. The errors that optical illusions induce in our perceptions are lawful, regular, and systematic. They are not dumb mistakes but smart mistakes—mistakes that allow those who understand them to glimpse the elegant design and inner workings of the visual system.

The mistakes we make when we try to imagine our personal futures are also lawful, regular, and systematic. They too have a pattern that tells us about the powers and limits of foresight in much the same way that optical illusions tell us about the powers and limits of eyesight. That's what this book is all about. Despite the third word of the title, this is not an instruction manual that will tell you anything useful about how to be happy. Those books are located in the self-help section two aisles over, and once you've bought one, done everything it says to do, and found yourself miserable anyway, you can always come back here to understand why. Instead, this is a book that describes what science has to tell us about how and how well the human brain can imagine its own future, and about how and how well it can predict which of those futures it will most enjoy. This book is about a puzzle that many thinkers have pondered over the last two millennia, and it uses their ideas (and a few of my own) to explain why we seem to know so little about the hearts and minds of the people we are about to become. The story is a bit like a river that crosses borders without benefit of passport because no single science has ever produced a compelling solution to the puzzle. Weaving together facts and theories from psychology, cognitive neuroscience, philosophy, and behavioral economics, this book allows an account to emerge that I personally find convincing but whose merits you will have to judge for yourself.

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Excerpted from Stumbling on Happiness by Daniel Gilbert Copyright © 2006 by Daniel Gilbert. Excerpted by permission of Knopf, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.