"Derech Hashem -- The Way of God" by Rabbi Moshe Chaim Luzzatto (Feldheim, 1997).
Quietly studying a page of the Talmud on a crowded plane, the great Orthodox teacher and thinker Rabbi Joseph B. Soloveitchik was interrupted by a passenger in the next seat.
"Pardon me. What is that you are studying?" the man asked.
Soloveitchik explained the nature of the Talmud, and that he was a professor of Talmud at Yeshiva University.
The man was incredulous. "Do you mean that people spend their entire lives thinking about religion?" he asked. "Why, I thought that all of religion could be succinctly summarized as 'Do unto others what you would have them do unto you'!"