Monday night’s US presidential debate should convince a majority of American voters that Hillary Clinton is their only credible choice for the White House. Yet it may well fail to do so, in the new era of ‘post-truth politics’. The historian Sir Michael Howard suggests that on both sides of the Atlantic, we are witnessing a retreat from reason, an attempt to reverse the onset of the 18th-century Age of the Enlightenment, which banished superstition and religious faith as a basis for reaching conclusions.