I just ran across another mind-blowing passage in Thomas Kuhn’s The Structure of Scientific Revolutions. He’s writing about why it is that some scientists are able see “anomalous” data — stuff that contradicts all known theories of science. Scientists, after all, frequently stare at anomalous data all day long — stuff that totally contradicts their ideas about the world. Yet they never really notice or pay attention to it. When you’re convinced of one way of looking at things, you see what you expect to see.

This was the basic idea behind a psychological experiment by J. S. Bruner and Leo Postman, “On the Perception of Incongruity,” published in the Journal of Personality in 1949. They took a deck of cards and altered some of the cards in subtle ways — like, for example, making a red six of spades, or a black four of hearts. Then they ran experiments, asking subjects to identify a set of cards — including a couple of the anomalous ones. They did not tell the subjects that some of the cards were anomalous, though. Thus:

Even on the shortest exposures 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 or 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 of the anomalous cards, they would have little further difficulty with the others.

Cool. But this is even more mind-blowing:

A few subjects, however, were never able to make the requisite adjustment of the 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 even look like a card that time. I don’t know what color it is now or whether it’s a spade or a heart. I’m not even sure what a spade looks like now. My god!” (pp. 63-64)