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Perception differs qualitatively from the physical properties of the stimulus. The nervous system extracts only certain information from the natural world. We perceive fluctuations of air pressure not as pressure waves but as sounds that we hear. We perceive electromagnetic waves of different frequency as colors that we see. We perceive chemical compounds dissolved in air or water as specific smells or tastes. In the words of neurologist Sir John Eccles: “I want you to realize that there exists no color in the natural world, and no sound – nothing of this kind; no textures, no patterns, no beauty, no scent.” Sounds, colors, patterns, etc., appear to have an independent reality, yet are, in fact, constructed by the mind. All our experience of the natural world is our minds interpretation of the input it receives.

The Soul Illusion

In a psychological experiment that deserves to be much better known outside the trade, Bruner and Postman asked experimental subjects to identify on short and controlled exposure, a series of playing cards. Many of the cards were normal, but some were made anomalous, e.g., a red six of spades and a black four of hearts. Each experimental run was constituted by the display of a single card to a single subject in a series of gradually increased exposures. After each exposure the subject was asked what he had seen, and the run was terminated by two successive correct identifications .

Even on the shortest exposure many subjects identified most of the cards, and after a small increase all the subjects identified them all, for the normal cards these identifications were usually correct, but the anomalous cards were almost always identified, without apparent hesitation or puzzlement, as normal. The black four of hearts might, for example, be identified as the four of either spades or hearts. without any awareness of trouble, it was immediately fitted to one of the conceptual categories prepared by prior experience. One would not even like to say that the subjects had seen something different from what they identified. With a further increase of exposure to the anomalous cards, subjects did begin to hesitate and to display awareness of anomaly. exposed, for example, to the red six of spades, some would say: that’s the six of spades, but there’s something wrong with it – the black has a red border. Further increase of exposure resulted in still more hesitation and confusion until finally, and sometimes quite suddenly, most subjects would produce the correct identification without hesitation. Moreover, after doing this with two or three anomalous cards, they would have little further difficulty with the others. A few subjects, however, were never able to make the requisite adjustment of their categories. Even at forty times the average exposure required to recognize normal cards for what they were, more than 10 per cent of the anomalous cards were not correctly identified. And the subjects who then failed often experienced acute personal distress.

One of them exclaimed: “I can’t make the suit out, whatever it is. It didn’t look like a card that time. I don’t know what color it is now or whether it is a spade or heart. I am not even sure now what a spade looks like. My God!” Thomas Kuhn – The Structure of Scientific Revolution

An experimental subject who puts on goggles fitted with inverting lenses initially sees the entire world upside down. At the start his perceptual apparatus functions as it had been trained to function in the absence of the goggles, and the result is extreme disorientation, an acute personal crisis. But after the subject has begun to learn to deal with his new world, his entire visual field flips over, usually after an intervening period in which vision is simply confused. Thereafter, objects are again seen as they had been before the goggles were put on. The assimilation of a previously anomalous visual field has reacted upon and changed the field itself. Literally as well as metaphorically, the man accustomed to inverting lenses has undergone a revolutionary transformation of vision.

What a man sees depends both upon what he looks at and also upon what his previous visual—conceptual experience has taught him to see.

The subject of a gestalt demonstration knows that his perception has shifted because he can make it shift back and forth repeatedly while he holds the same book or piece of paper in his hands. Aware that nothing in his environment has changed, he directs his attention increasingly not to the figure [duck or rabbit] but to the lines on the paper he is looking at. Ultimately he may even learn to see those lines without seeing either of the figures, and he may then say (what he could not legitimately have said earlier) that it is these lines that he really sees but that he sees them alternately as a duck and as a rabbit. By the same token, the subject of the anomalous card experiment knows (or, more accurately, can be persuaded) that his perception must have shifted because an external authority, the experimenter, assures him that regardless of what he saw, he was looking at a black five of hearts all the time. In both these cases, as in all similar psychological experiments, the effectiveness of the demonstration depends upon its being analyzable in this way. Unless there is an external standard with respect to which a switch of vision could be demonstrated, no conclusion about alternate perception could be drawn.

Thomas Kuhn – The Structure of Scientific Revolution