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Steven Pinker -- How the Mind Works - Hampshire High Italian ...

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28 HOW THE MIND WORKSmechanisms. The organ systems of <strong>the</strong> body do <strong>the</strong>ir jobs because eachis built with a particular structure tailored to <strong>the</strong> task. The hfeart circulates<strong>the</strong> blood because it is built like a pump; <strong>the</strong> lungs oxygenate <strong>the</strong>blood because <strong>the</strong>y are built like gas exchangers. The lungs cannotpump blood and <strong>the</strong> heart cannot oxygenate it. This specialization goesall <strong>the</strong> way down. Heart tissue differs from lung tissue, heart cells differfrom lung cells, and many of <strong>the</strong> molecules making up heart cellsdiffer from those making up lung cells. If that were not true, our organswould not work.A jack-of-all-trades is master of none, and that is just as true for ourmental organs as for our physical organs. The robot challenge makes thatclear. Building a robot poses many software engineering problems, anddifferent tricks are necessary to solve <strong>the</strong>m.Take our first problem, <strong>the</strong> sense of sight. A seeing machine mustsolve a problem called inverse optics. Ordinary optics is <strong>the</strong> branch ofphysics that allows one to predict how an object with a certain shape,material, and illumination projects <strong>the</strong> mosaic of colors we call <strong>the</strong> retinalimage. Optics is a well-understood subject, put to use in drawing,photography, television engineering, and more recently, computer graphicsand virtual reality. But <strong>the</strong> brain must solve <strong>the</strong> opposite problem. Theinput is <strong>the</strong> retinal image, and <strong>the</strong> output is a specification of <strong>the</strong> objectsin <strong>the</strong> world and what <strong>the</strong>y are made of—that is, what we know we areseeing. And <strong>the</strong>re's <strong>the</strong> rub. Inverse optics is what engineers call an "illposedproblem." It literally has no solution. Just as it is easy to multiplysome numbers and announce <strong>the</strong> product but impossible to take a productand announce <strong>the</strong> numbers that were multiplied to get it, optics iseasy but inverse optics impossible. Yet your brain does it every time youopen <strong>the</strong> refrigerator and pull out a jar. <strong>How</strong> can this be? !The answer is that <strong>the</strong> brain supplies <strong>the</strong> missing information, informationabout <strong>the</strong> world we evolved in and how it reflects light. If <strong>the</strong> Visual brain"assumes" that it is living in a certain kind of world—an evenly lit worldmade mostly of rigid parts with smooth, uniformly colored surfaces—it canmake good guesses about what is out <strong>the</strong>re. As we saw earlier, it's impossibleto distinguish coal from snow by examining <strong>the</strong> brightnessies of <strong>the</strong>irretinal projections. But say <strong>the</strong>re is a module for perceiving <strong>the</strong> properties ofsurfaces, and built into it is <strong>the</strong> following assumption: "The world issmoothly and uniformly lit." The module can solve <strong>the</strong> coal-versus-snowproblem in three steps: subtract out any gradient of brightness from oneedge of <strong>the</strong> scene to <strong>the</strong> o<strong>the</strong>r; estimate <strong>the</strong> average level of brightness of

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