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

242 | HOW THE MIND WORKSKepler, Wheatstone, and aerial reconnaissance engineers. It evolved inresponse to identifiable selection pressures in the ecology of our ancestors.And it solves unsolvable problems by making tacit assumptionsabout the world that were true when we evolved but are not always truenow.LIGHTING, SHADING. SHAPINGStereo vision is part of a crucial early stage of vision that figures out thedepths and materials of surfaces, but it is not the only part. Seeing inthree dimensions doesn't require two eyes. You can get a rich sense ofshape and substance from the meagerest hints in a picture. Look at thesedrawings, designed by the psychologist Edward Adelson.The left one appears to be white cardboard with a gray vertical stripe,folded horizontally and lit from above. The right one appears to be whitecardboard with a gray horizontal stripe, folded vertically and lit from theside. (If you stare long enough, either might flip in depth, like a Neckercube; let's ignore that for now.) But the ink on the page (and the projectionon your retina) is virtually the same in the two pictures. F.ach is azigzag tic-tac-toe box with some of the squares shaded in. In both drawings,the comer squares are white, the top and side squares are light gray,and the middle square is a darker gray. Somehow the combination ofshading and zigzagging pops them into the third dimension and colorizeseach square, but in different ways. The borders labeled "1" are physicallythe same in the two drawings. But in the left drawing the border lookslike a paint boundary—a white stripe next to a gray one—and in the rightdrawing it looks like a shape-and-shading boundary—a white stripefalling into a shadow on the other side of a fold. The borders labeled "2"are also identical, but you see them in the opposite way: shadow in the