Today most professional historians ‘specialise’. They choose a period, sometimes a very brief period, and within that period they strive, in desperate competition with ever-expanding evidence, to know all the facts. Thus armed, they can comfortably shoot down any amateurs who blunder… into their heavily fortified field… Theirs is a static world. They have a self-contained economy, a Maginot Line and large reserves… but they have no philosophy. For historical philosophy is incompatible with such narrow frontiers. It must apply to humanity in any period. To test it, a historian must dare to travel abroad, even in hostile country; to express it he must be ready to write essays on subjects on which he may be ill-equipped to write books.
Hugh Trevor-Roper, quoted in Norman Davies, “Vanished Kingdoms: The Rise and Fall of States and Nations”