Answer: The person who said "Leave him [Emile] at liberty and by himself, and without saying a word, watch what he does, and how he does it. Knowing perfectly well that he is free, he will do nothing from mere thoughtlessness..." was Jean-Jacques Rousseau.
Explanation: Jean-Jacques Rousseau includes the words provided in one of his most famous treatises: Emile, or On Education. In this piece of writing, Rousseau deals with the subject of education and highlights the importance of education in a corrupt society. Emile, or On Education had an important influence on the design of the new system of education in France after 1789.