The state of slavery is in its own nature bad. It is neither useful to the master nor to the slave; not to the slave, because he can do nothing through a motive of virtue; nor to the master, because by having an unlimited authority over his slaves he insensibly accustoms himself to the want of all moral virtues, and thence becomes fierce, hasty, severe, choleric, voluptuous, and cruel. … where it is of the utmost importance that human nature should not be debased or dispirited, there ought to be no slavery.

De l'Esprit des Lois (1748) [The Spirit of the Laws]