4 ideas
An outsider describes familiar customs literally, without knowing why they exist, and this strips away the habit that makes them seem natural so their arbitrariness shows. Rica calls the Pope a magician who makes people believe three are one and bread is not bread. He calls the King a magician who makes people believe paper is money.
Usbek, who reasons eloquently about justice and liberty in Paris, rules his wives in Isfahan through eunuchs, surveillance and threats sent by letter. In his long absence the harem falls into disorder, and he orders savage punishments by post. The novel ends with his favourite wife Roxane poisoning herself, after writing that she had always been free in her heart and had deceived him. It is a concrete case of an enlightened man practising the despotism he criticises, and of rule by fear failing once the ruler is distant.
Rule by fear alone gets outward obedience, not loyalty. It destroys the trust and inner consent that would let authority survive the despot's absence. Agents given unchecked power, like the eunuchs, become tyrants in their turn.
Usbek tells of the Troglodytes, a people who each pursued only self-interest. They destroyed one another through theft, betrayal and refusing mutual aid until almost none were left. The surviving virtuous families flourished by treating justice to others as their own interest. When their descendants later asked for a king, they were giving up the burden of virtue for the ease of obeying laws. The fable argues that a society's survival depends on citizens internalising justice, not just being coerced by rules.