Away from the frontiers and mythology of Enlightenment, the meaning of ‘rationality’ (and hence ‘irrationality’) becomes difficult to pin down. You can resort to the otherworldly ideas of logic and mathematics floating free from all politics and culture. But the academic study of ‘rational choice’ makes little sense once diverted from the kinds of strategic problem – war and profit – it has long been tasked with solving. When we reflect on how we actually live, it becomes all the harder to identify what an ‘irrational’ action or choice might be. Smith wonders ‘whether an anthropologist external to our cultural world would, in studying us, be able to make sharp distinctions among the horoscope, the personality quiz and the credit rating’, or even be able to tell ‘whether we ourselves clearly understand how they differ’. Equally, it isn’t clear how one would distinguish between the scientific societies of the 17th century, to which so much subsequent progress is owed, and, say, a website dedicated to picking through the evidence that vaccines cause autism. Understood purely as ‘culture’ or as ‘behaviour’, rationality becomes ritual or (as the nudgers have it) habit, and ‘irrationality’ is just a pejorative term for the habits we consider bad.