Lecturing at the time of the Franco-Prussian war, Jacob Burckhardt told his history students that the revolutionary age from 1789 onwards had been lehrreich: rich in teaching. The greatest spirits of a hundred years before were now looking short of knowledge, but only because of what had happened since. Modern students should not attribute virtue to themselves just because they could see so much that their mental superiors had not foretold. It is worth remembering Burckhardt's principle when we come to deal with another great lecturer, Isaiah Berlin. He was once famous for understanding everything about the age he lived in. There is still reason to believe he understood a lot. But if today he is starting to look a bit less penetrating about it all, it could be because things have moved on.