The Simple Logical Puzzle That Shows How Illogical We Are

Brian Gallagher in Nautilus:

ScreenHunter_2350 Nov. 07 08.48In the 1960s, the English psychologist Peter Wason devised an experiment that would revolutionize his field. This clever puzzle, known as the “Wason selection task,” is often claimed to be “the single most investigated experimental paradigm in the psychology of reasoning,” in the words of one textbook author.

Wason was a funny and clever man and an idiosyncratic thinker. His great insight was to treat reasoning as an enigma, something to scrutinize both critically and playfully. He told his colleagues, for instance, that he would familiarize himself with their work only after doing his own experiments, so as not to bias his own mind. He also said that before running experiments, researchers—quixotically—should never really know exactly why they were doing them. “The purpose of his experiments was not usually to test a hypothesis or theory, but rather to explore the nature of thinking,” a pair of his students wrote in Wason’s obituary. (He died in 2003.) “His aim was to reveal a surprising phenomenon—to show that thinking was not what psychologists including himself had taken it to be.”

The groundbreaking nature of Wason’s selection task may have been a result of his unconventional style. In one version of the task, one subject (always one—he spurned testing subjects in groups) is presented with four cards lying flat on a table, each with a single-digit number on one face and one of two colors on the other. Let’s imagine that you’re Wason’s subject. The first and second cards you see are a five and an eight; the third and fourth cards are blue and green, respectively. Wason liked to chat with his subjects, but he probably didn’t tell them that this logical puzzle was “deceptively easy,” which was how he described it in the paper he would later write, in 1968.

Wason tells you that if a card shows an even number on one face, then its opposite face is blue. Which cards must you turn over in order to test the truth of his proposition, without turning over any unnecessary cards?

More here.