Final answer:
Gibbon and Diderot agree that a necessary duty of man is to contribute to the happiness and well-being of all people, encouraging universal benevolence, education, and the cultivation of moral virtues.
Step-by-step explanation:
Historical figures such as Gibbon and Diderot observed that a key duty of man is to work towards the happiness and well-being of all humanity, transcending divisions such as class, sect, party, country, or color. This principle reflects a commitment to universal benevolence and the promotion of a society where the general welfare is paramount. Enlightenment philosophies like those espoused by Diderot emphasize the power of education and art in shaping moral virtues and civic responsibility. The idea prevails that by cultivating such values as a society, individuals find their highest fulfillment.