The lecture is based around the concept of natural man which emerged and developed during the Age of Enlightenment.
As a term implying something obviously natural and normal, “essential” attained a special meaning in the fierce debates between the key figures of the Enlightenment [I’d rather say European, bc Rousseau was Swiss, and there were other non-French guys, e.g. Hume, or Kant] who saw the “natural man” as a clue for understanding social processes. Whereas Rousseau believed that the natural source of human soul is pure and could only be ruined by culture and civilization, Voltaire regarded the “essential” origin of human nature as the basis of social disasters. These debates have led to the emergence of such categories as “natural order”, “natural law”, and “natural religion”. Traces left by these ideas can still be found in modern science.
“In the natural order men are all equal and their common calling is that of manhood, so that a well-educated man cannot fail to do well in that calling and those related to it. It matters little to me whether my pupil is intended for the army, the church, or the law. Before his parents chose a calling for him nature called him to be a man. Life is the trade I would teach him. When he leaves me, I grant you, he will be neither a magistrate, a soldier, nor a priest; he will be a man. All that becomes a man he will learn as quickly as another. In vain will fate change his station, he will always be in his right place.”
(Jean-Jacque Rousseau. Emile, or On Education. 1762)