Fifty years on, some experts on reasoning – working in economics, philosophy and psychology – still respond to evidence of human irrationality with denial. They either ignore the evidence altogether, or insist that it does no more than find rare exceptions to the rational norm. According to this traditional view, as long as we are not misled by vanity, emotion, or tricky ways of posing the question, we do actually reason in the way we should reason – as prescribed by probability theory, decision theory and logic, for example – and via mental processes that are captured pretty well by formalisms such as: All As are Bs, All Cs are As, Therefore all Cs are Bs. If we know (or are told by Aristotle) that All humans are mortal and All Greeks are human, we can compose sentences in our heads where “humans” appears in the “A” slot, “mortal” in the “B” slot, and “Greeks” in the “C”slot, and then turn a handle marked “deductive syllogism” to discover that All Greeks are mortal. And we are mighty good at performing this kind of operation. On the traditional view, reasoning is, as Hugo Mercier and Dan Sperber put it, a human “superpower” that sets us apart from all other animals.
But the majority of experts are no longer in denial. Instead they respond to evidence of irrationality with deflection, duplication, dedication, or despair. The deflection response suggests that reasoning conforms to non-standard principles. Perhaps the logical standards we meet, or even the processes in our heads, are not those devised by Aristotle. The duplicators – most famously, Daniel Kahneman in his book Thinking, Fast and Slow – believe there are two psychological systems involved in answering questions such as “Is Linda more likely to be a bank teller, or a bank teller and an active feminist?”
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