Slavery is not good in itself: it is neither useful to the master nor to the slave, because the slave can do nothing from virtuous motives; nor to the master, because he contracts amongst his slaves all sorts of bad habits — he becomes haughty, passionate, obdurate, vindictive, voluptuous, and cruel.

[Il n’est pas bon par sa nature; il n’est utile ni au maître ni à l’esclave: à celui-ci, parce qu’il ne peut rien faire par vertu; à celui-là, parce qu’il contracte avec ses esclaves toutes sortes de mauvaises habitudes, qu’il s’accoutume insensiblement à manquer à toutes les vertus morales, qu’il devient fier, prompt, dur, colère, voluptueux, cruel.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

The Spirit of Laws [De l’esprit des lois], Vol. 1, Book 15, ch. 1 (1748)

(Source (French)).

Common translation used by English and American abolitionists (e.g., 1812). Alternate translations:

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; not to the master, because by having an unlimited authority over his slaves, he insensibly accustoms himself to the want of all moral virtues, and from thence grows fierce, hasty, severe, choleric, voluptuous, and cruel.
[tr. Nugent (1758 ed.)]

It is not good by its nature; it is useful neither to the master nor to the slave: not to the slave, because he can do nothing from virtue; not to the master, because he contracts all sorts of bad habits from his slaves, because he imperceptibly grows accustomed to failing in all the moral virtues, because he grows proud, curt, harsh, angry, voluptuous, and cruel.
[tr. Cohler/Miller/Stone (1989)]