Question

how do historians divide the past into periods? Do they face any problemsin doing so?

Answer 1

They divide the past into periods because by doing so it becomes easy for them to record the different events taking place in those times.
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Answer 2

Historians divide the past into periods based on the economic and social factors which characterize them. In doing so they are faced with two problems. First, economic and social changes keep taking place hence definite boundaries cannot be drawn. Second, these periods are compared with modernity.