Lewis Fry Richardson, a Quaker who had driven ambulances in the First World War, spent the years after it trying to find out mathematically why countries go to war. One idea was that neighbours with longer shared borders fight more often, so he went to look up the lengths. Portugal said its border with Spain was 987 kilometres. Spain said 1,214. Neither had made a mistake. They had measured with different rulers, and on a wiggly line a shorter ruler fits into more of the wiggles, so the total keeps growing the closer you look. A border like that has no true length. Its length depends on how carefully you measure it. Richardson died in 1953 without finding his law of war. His notes on the rulers came out in 1961, and in 1967 Benoit Mandelbrot built on them in a paper called “How Long Is the Coast of Britain?”, which was a starting point for fractals. So the man who went looking for what divides countries found that the line between them can’t be measured, and I think he would have liked that.