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

234 I HOW THE MIND WORKSDavid Marr noted that built-in assumptions about the world weevolved in can come to the rescue. Among the n 2 possible matches of npoints, not all are likely to have come from this goodly frame, the earth.A well-engineered matcher should consider only the matchups that arephysically likely.First, every mark in the world is anchored to one position on one surfaceat one time. So a legitimate match must pair up identical points inthe two eyes that came from a single splotch in the world. A black dot inone eye should match a black dot in the other, not a white dot, becausethe matchup has to represent a single position on some surface, and thatposition cannot be a black splotch and a white splotch at the same time.Conversely, if a black dot does match a black dot, they must come from asingle position on some surface in the world. (That is the assumptionviolated by autostereograms: each of their splotches appears in severalpositions.)Second, a dot in one eye should be matched with no more than onedot in the other. That means that a line of sight from one eye is assumedto end at a splotch on one and only one surface in the world. At firstglance it looks as if the assumption rules out a line of sight passingthrough a transparent surface to an opaque one, like the bottom of ashallow lake. But the assumption is more subtle; it only rules out thecoincidence in which two identical splotches, one on the lake's surfaceand one on the bottom, line up one behind the other from the left eye'svantage point while both being visible from the right eye's.Third, matter is cohesive and smooth. Most of the time a line of sightwill end up on a surface in the world that is not drastically closer or fartherthan the surface hit by the neighboring line of sight. That is, neighboringpatches of the world tend to lie on the same smooth surface. Ofcourse, at the boundary of an object the assumption is violated: the edgeof the back cover of this book is a couple of feet away from you, but ifyou glance just to its right you might be looking at the moon a quarter ofa million miles away. But boundaries make up a small portion of thevisual field (you need much less ink to sketch a line drawing than tocolor it in), and these exceptions can be tolerated. What the assumptionrules out is a world made up of dust storms, swarms of gnats, fine wires,deep crevasses between craggy peaks, beds of nails viewed point-on, andso on.The assumptions sound reasonable in the abstract, but somethingstill has to find the matches that satisfy them. Chicken-and-egg prob-