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They have not only explained the development of human society upto the present stage, they have discovered the ways and means of attaining the ideal society of human being.'The success of one is the precondition of the success of the other'.
They concluded that 'the history of all the hithrto existing societies is the history of class struggles. Freeman and slave, patrician and plebian, lord and serf, guild master and journey man, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on an uninterrupted, now hidden, now open fight, a fight that each time ended , either in a revolutionary re-constitution of society at large, or in the common ruin of the contending classes.
So far as the documents revealed after excavation of Mohenjodaro-Harappa civilisation, it was found that in Indus Valley civilisation the division of classes existed. Those who had different opinions, would have no prove of documents.
In the Indus valley civilisation excavations proved the existence of unequal status between different members of the societies.The rich persons lived in multistoried building where as the poor people lived in small thatched rooms. Even the graveyards showed the difference between a rich and a poor person. In the cities of Harappa age there were slaves. The class division in the time of Aryans had seeds in the Harappa civilisation.