The civilisation of ancient Greece was nurtured within city walls. In fact, all the modern civilisations have their cradles of brick and mortar.
These walls leave their mark deep in the minds of men. They set up a principle of "divide and rule" in our mental outlook, which begets in us a habit of securing all our conquests by fortifying them and separating them from one another. We divide nation and nation, knowledge and knowledge, man and nature. It breeds in us a strong suspicion of whatever is beyond the barriers we have built, and everything has to fight hard for its entrance into our recognition.
When the first Aryan invaders appeared in India it was a vast land of forests, and the new-comers rapidly took advantage of them. These forests afforded them shelter from the fierce heat of the sun and the ravages of tropical storms, pastures for cattle, fuel for sacrificial fire, and materials for building cottages.