Montesquieu

Here is ano­ther ori­gin of the right of sla­very, and even of the cruel kind of sla­very we see among men.

There are coun­tries where heat ener­va­tes the body, and so saps cou­rage that men are moti­va­ted to a hard duty only by fear of punish­ment ; sla­very the­re­fore cla­shes less with rea­son ; and the mas­ter being as cowardly with res­pect to his prince as the slave is with res­pect to him, civil sla­very is fur­ther accom­pa­nied by poli­ti­cal sla­very.

Aristotle attempts to prove that there are sla­ves by nature, and what he says does not prove it at all.1 I think that if such exist, they are those I have just men­tio­ned.

But as all men are born equal, we must say that sla­very is unna­tu­ral, although in cer­tain coun­tries it is based on a natu­ral rea­son ; and we really must dis­tin­guish those coun­tries from others where even natu­ral rea­sons reject it, like the nations of Europe where it has so hap­pily been abo­li­shed.

Plutarch tells us in the life of Numa that in the time of Saturn there was nei­ther mas­ter nor slave. In our cli­ma­tes, Christianity has brought that era back.

Politics, book I, ch. i.