This raises a tough question. The ability to reason is meant to be humanity’s supreme attribute, the characteristic that most sets us apart from other animals. Why, then, has evolution endowed us with a tool so faulty that, if you bought it from a shop, you’d send it back? The French evolutionary psychologists Hugo Mercier and Dan Sperber have offered an intriguing answer to this question. If our reasoning capacity is so bad at helping us as individuals figure out the truth, they say, that’s because truth-seeking isn’t its function. Instead, human reason evolved because it helps us to argue more effectively.