That there should be a match between perception and reality is not surprising, because evolution ruthlessly eliminates the unfit. If you routinely misperceive or even hallucinate and act on those misapprehensions, you won’t survive long in a world filled with dangers whose avoidance requires accurate distance and speed assessments and rapid reactions. Whether you are diving into rocky waters or driving on a narrow, two-lane road with cars whizzing by in the opposite direction, small mistakes can be lethal.
You probably believe that your eyes register high-fidelity information about the absolute size, speed and distance of visible objects and that you respond based on these impartial data. But although we build robots in this manner, equipping them with sensors and computers to plumb the metric properties of their environments, evolution has taken a more complex route.
As psychologists and neuroscientists have discovered over the past several decades, our consciousness provides a stable interface to a rich sensory world. Underneath this interface lurk two vision systems that work in parallel. Both are fed by the same two sensors, the eyeballs, yet they serve different functions. One system is responsible for visual perception and is necessary for identifying objects such as approaching cars and potential mates independent of their apparent size or location in our visual field. The other is responsible for action: it transforms visual input into the movements of our eyes, hands and legs. We consciously experience only the former, but we depend for our survival on both.