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

244 | HOW THE MIND WORKSabout projection (how objects appear in perspective} and someprobabilities about the world (what kinds of objects it has). Some ofthe probabilities about projection are very good indeed. A penny, theoretically,can project to a thin line, but it does so only when il is viewededge-on. If there's a penny in the scene, what is the probability that youare viewing it edge-on? Unless someone has choreographed the two ofyou, not very high. The vast majority of viewpoints will make the pennyproject an ellipse instead. The shape analyzer assumes the current viewpointis generic—not poised with pinpoint accuracy to line things up,Ames-style—and places its chips accordingly. A matchsiick, on the otherhand, will project to a straight line almost all the time, so if there is a linein an image, a stick is a better guess than a disk, all else being equal.A collection of lines in an image can narrow the odds even further, forexample, a set of parallel or near-parallel lines is seldom an accident.Nonparallel lines in the world rarely project near-parallel lines in animage: most pairs of sticks strewn on a floor cross at moderate to sharpangles. But lines that are parallel in the world, such as the edges of atelephone pole, almost always project near-parallel lines. So if there arenear-parallel lines in an image, the odds favor parallel edges in the world.There are many other rules of thumb that say what kinds of seulptings ofthe world can be counted on to give off various markings in an image.Little Ts, Ys, angles, arrows, crows' feet, and parallel wiggles are the fingerprintsof various straight edges, corners, right angles, and symmetricalshapes. Cartoonists have exploited these rules for millennia, and a wilyshape analyzer can run them backwards when betting on what is in theworld.But of course running a likelihood backwards—saying that parallelstuff usually projects near-parallel images, therefore near-para lie Iimages imply parallel stuff—is unsound. It is like hearing hoofbeats outsideyour window and concluding that they came from a zebra, becausezebras often make hoofbeats. The prior probability that the world containssome entity—how many zebras are out there, how many paralleledges are out there—has to be multiplied in. For an odds-playing shapeanalyzer to work, the world had better contain lots of the straight, regular,symmetrical, compact kinds of objects that it likes to guess. Does it?A romantic might think that the natural world is organic and soft, itshard edges bulldozed in by the Army Corps of Engineers. As a literatureprofessor recently declared to his class, "Straight lines on the landscapeare put there by man." A skeptical student, Gail Jensen Sanford, pub-