Question

why and how do we periodise a time of history

Answer 1

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Indus Valley Civilization- this one is self-explanatory.

Early Vedic. This would comprise from the rigvedic times uptil the rise of the great empires of Nanda and Maurya. So roughly around 1000 years. This would be home to composition of various shrutis and smritis, as well as early developments in the avaidika faiths like bauddha, jaina, avijika, etc. From early settlements the landscape would start sprouting cities and city-kingdoms and then bigger empires.

Classical Age: This would probably be the biggest slice out of the previous slice. Roughly from the onset of the Maurya Empire, which covered massive tracts of the subcontinent, to the demise of the last of the classical kingdoms of Yadavas, Kakatiyas, etc to the emergent Sultanates.

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Answer 2

James Mill divided the Indian history into three periods: Hindu, Muslim and British.

According to  Mill, Only British rules, culture and laws could make indians civilised.

There are significant sources to study the periods of events and by studing different historical evidences a string of time periods can be formed.