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

258 J HOW THE MIND WORKSis not just a reminder that you are not Superman. We mortals could havebeen equipped with a photographic memory that updates a 3-D modelby pasting in information from previous views wherever it belongs. Butwe were not so equipped. When it comes to rich visual detail, out ofsight is out of mind.Third, we see in perspective. When you stand between railroad tracks,they seem to converge toward the horizon. Of course you know they donot really converge; if they did, the train would derail. But it's impossiblenot to see them as converging, even though your sense of depth providesplenty of information that your brain could use to cancel the effect. Wealso are aware that moving objects loom, shrink, and foreshorten. In agenuine scale model, none of this can happen. To be sure, the visual systemeliminates perspective to a certain degree. People other than artistshave trouble seeing that the near corner of a desk projects an acute angleand the far corner an obtuse angle; both look like the right angles they arein reality. But the railroad tracks show that perspective is not completelyeliminated.Fourth, in a strict geometric sense we see in two dimensions, notthree. The mathematician Henri Poincare came up with an easy way todetermine the number of dimensions of some entity. Find an object thatcan divide the entity into two pieces, then count the dimensions of thedivider and add one. A point cannot be divided at all; therefore, it has zerodimensions. A line has one dimension, because it can be severed by apoint. A plane has two dimensions, because it can be rent by a line,though not by a point. A sphere has three, because nothing less than atwo-dimensional blade can cleave it; a pellet or a needle leaves it whole.What about the visual field? It can be sundered by a line. The horizon, forexample, divides the visual field in two. When we stand in front of a tautcable, everything we see is on one side or the other. The perimeter of around table also partitions the visual field: every point is either within it oroutside. Add one to the one-dimensionality of a line, and you get two. Bythis criterion, the visual field is two-dimensional. Incidentally, this doesnot mean that the visual field is flat. Two-dimensional surfaces can becurved in the third dimension, like a rubber mold or a blister package.Fifth, we don't immediately see "objects," the movable hunks of matterthat we count, classify, and label with nouns. As far as vision is concerned,it's not even clear what an object is. When David Marrconsidered how to design a computer vision system that finds objects, hewas forced to ask: