This makes nostalgia dangerous. Protesters do not necessarily expect to put back the clock—they may just be seeking to slow it down. And yet such sclerosis may only aggravate the sense of decline. In addition, the self-esteem that nostalgists crave often seems to feed xenophobia. India’s Hindu-nationalist revival has seen an increase in reported hate crimes towards Muslims. In the West people on the right remember a whiter past, with fewer cultures, even as the hard left condemns the machinations of global business. It is no accident that there has been a resurgence of anti-Semitism, not least in Britain’s Labour Party under the backward-looking Jeremy Corbyn.


Because of the taint of xenophobia, progressives are quick to treat all nostalgia as prejudice, leading them to dismiss the fears of whole sections of society. That sweeping judgment is one more reason why populists have been able to exploit nostalgia so successfully.


They are having it too easy. Nostalgia can be harnessed for good. At the start of the 20th century, Europe and America were nostalgic, too, buffeted by a similar confluence of technological, geopolitical and cultural change. Then a period of conflict and social upheaval led to universal suffrage and education for all. Today’s politicians can learn from that time. They must avoid war, obviously, by preserving and enhancing the institutions that enable countries to work together. But they should also find bold ways to deal with insecurity and alienation. That will involve the state working harder for the citizen by making education available throughout people’s lives, by overhauling taxation, devolving power to cities and regions, averting climate catastrophe, and wise management of immigration.


The nostalgists are on to something. When one way of running the world seems to be exhausted, but the next has yet to come into being, the past holds important lessons. When nothing seems to make sense, history becomes the supreme discipline. Knowing who you are and where you came from matters.


The best way to harness the past demolishes prejudice and opens horizons. A proper sense of history helps you grasp that progress depends on facing up to hard choices. Sometimes it can inspire, too. Fifty years ago Apollo 8 took off from Cape Canaveral in Florida. On December 24th it captured a photograph of Earth, a half-shrouded blue-white planet, seemingly united.