“Categorical imperative” refers to Immanuel Kant’s conception of an absolute moral principle, binding upon the individual in all circumstances. In short, it demands that each person regard every other person as an end unto themselves, never to be treated as a means to someone else’s end. Everyone must regard everyone else as equally morally important to themselves. This is not a principle that requires logical justification or divine sanction—it receives sufficient backing from the fact that adhering to this principle would be of obvious benefit to humanity, while violating it is just as obviously harmful.

“The Enlightenment” refers to a period in Western philosophy and culture roughly covering the 17th and 18th centuries. In Durant’s book, Voltaire is the embodiment of the main ideals of this movement. Emerging out of a period of vicious religious warfare, the Enlightenment shifted focus from worship of God to celebration of the human person, particularly beauty and reason. It used these ideas to challenge traditional forms of authority, especially aristocratic and ecclesiastical hierarchies, and would help pave the way for modern democratic concepts of every person holding the potential for wisdom and the right to civic participation.