Synthesized answer
The provided passages do not contain the answer to what the central thesis of "Letters for the Advancement of Humanity (1793-1797)" is. The passages only indicate the title and page count of a different work, "Early and Miscellaneous Letters by Johann Wolfgang von Goethe" [Passage 1]. There is no information presented about "Letters for the Advancement of Humanity (1793-1797)" itself.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Early and Miscellaneous Letters by Johann Wolfgang von Goethe Pages: 440