E. H. Gombrich on perception

Even our natural view of the world is theoretical …

From Art and Illusion by E. H. Gombrich:

Just as a tune remains the same whatever the key it is played in, so we respond to light intervals, to what have been called ‘gradients’, rather than the measurable quantity of light reflected from any given object. And when I say ‘we’, I include newly hatched chickens and other fellow-creatures who so obligingly answer the questions psychologists put to them. According to a classic experiment by Wolfgang Köhler, you can take two grey pieces of paper — one dark, one bright — and teach the chickens to expect food on the brighter of the two. If you then remove the darker piece and replace it by one brighter than the other one, the deluded creatures will look for their dinner, not on the identical grey paper where they have always found it, but on the paper where they would expect it in terms of relationships — that is, on the brighter of the two. Their little brains are attuned to gradients rather than to individual stimuli. Things could not go well with them if nature had willed it otherwise. For would a memory of the exact stimulus have helped them to recognize the identical paper? Hardly ever! A cloud passing over the sun would hang its brightness, and so might even a tilt of the head, or an approach or from a different angle. If what we call ‘identity’ were not anchored in a constant relationship with environment, it would be lost in the chaos of swirling impressions that never repeat themselves.