Periodization, dividing the past into periods that can be clearly iden-
tified and that differ from one another in a recognizable way, is a 
subject for interminable discussion.Nevertheless, periodization, however arbitrary and subject to the 
personal preferences of the historian, is an unavoidable and even indis-
pensable tool to give shape to the past, which would otherwise consist 
of an undifferentiated mass of facts and figures.Like com-
parisons, periodizations in principle are unlimited in number, but they 
only serve a purpose if they allow us to partition the stream of events in 
such a way that important developments become visible.This begs the 
question of which developments the historian sees as important enough 
to warrant basing his periodization on, or in other words, which among 
the great mass of facts he recognizes as 'historical facts'.Of course, in any given field there are traditional divisions that have 
become so widespread that the innocent reader tends to accept them as 
historical facts in themselves, not to say natural phenomena.First, it must have explanatory value.