There must be much less difference between one man and another in the state of nature than in that of society, and natural inequality must increase in the human species through inequality occasioned by social institutions.
Gist of Idea
In a state of nature people are much more equal; it is society which increases inequalities
Jean-Jacques Rousseau (Discourse on the Origin of Inequality , Part I)
This is the main idea of his essay - the answer to the question set by the essay prize. Slavery is common in fairly basic societies, but that is at a much more advanced stage than Rousseau is thinking of. It's hard to disagree with him.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.58
Idea 19786 Three stages of the state produce inequalities of wealth, power, and enslavement [Rousseau]