“What is a man,” asks Hamlet, if he leaves his “god-like reason / To fust … unused?”

“Not much,” answers Steven Pinker, who begins Rationality (Allen Lane, £25; Tablet price £22.50) by quoting the Bard. Against a background of fake news, post-truth and conspiracy theories, the famously optimistic Harvard psychologist sets out to show that humans are rational animals and that the popular justification for our irrationality – we are basically cavemen in clothes – is neither true nor helpful. We can think better and Pinker offers readers a toolkit to do so.