Montesquieu

Slavery, pro­perly spea­king, is the ins­ti­tu­tion of a right that makes one man so behol­den to ano­ther man that that man is the abso­lute mas­ter of his life and pro­perty. It is not good by its nature ; it is use­ful nei­ther to the mas­ter nor to the slave : to the slave, because he can do nothing out of vir­tue ; to the mas­ter, because he contracts all sorts of bad habits with his sla­ves, because he accus­toms him­self lit­tle by lit­tle to fai­ling in all the moral vir­tues, and because he beco­mes proud, impe­tuous, mean, conten­tious, sen­suous, and cruel.

In des­po­tic coun­tries which are already under poli­ti­cal sla­very, civil sla­very is more tole­ra­ble than elsew­here. Everyone must be content enough to have his sub­sis­tence and his life. Thus the condi­tion of the slave is scar­cely more heavy than the condi­tion of the sub­ject.

But in the monar­chi­cal govern­ment, where it is supre­mely impor­tant not to crush or demean human­kind, there must be no slave. In demo­cracy, where eve­ryone is equal, and in aris­to­cracy, where the laws must attempt to make eve­ryone as equal as the nature of the govern­ment can allow, sla­ves are contrary to the spi­rit of the cons­ti­tu­tion ; they serve only to give citi­zens an autho­rity and a luxury they ought not to have.