Montesquieu
To prevent abuse, power must check power.
Misconceptions
Montesquieu invented the separation of powers.
The phrase "separation of powers" is not his. Nor is the idea new: Locke, in 1690, already explains that it is dangerous for those who make the laws to execute them too. What Montesquieu adds is a formula and a mechanism: "power checks power". He distinguishes three powers (making laws, executing them, judging), but does not isolate them: each can stop the other, like the king who can reject a law. It is a balance more than a separation.
Montesquieu justifies slavery.
False for the most quoted chapter. "On the slavery of the negroes" (XV, 5) is ironic: it opens with "Were I to vindicate our right [...], these should be my arguments", then lists deliberately absurd arguments to ridicule the slave owners. Quoting one of those sentences alone makes him say the opposite of what he thinks. His thesis is elsewhere: slavery "is in its own nature bad" (XV, 1) and, "as all men are born equal", it is "unnatural" (XV, 7). But there is a real grey zone: in the same place, his climate theory makes him grant that in some very hot countries slavery is "founded on natural reason". He then steps back (XV, 8): there is "possibly" no climate where free men could not be made to work. Scholars still debate this concession.
For Montesquieu, climate decides everything.
For him climate is only one cause among others (religion, laws, customs, manners), and good legislators are those who fight its bad effects. But his climate theory rests on a period physiology ("fibres" that tighten in the cold) and on travellers' tales. Some sentences are now dated and shocking, such as "The power in Asia ought to be always despotic". It is an 18th-century thesis, not a fact.