Explain What Montesquieu means when he writes every man invested with power is apt to abuse it and carry his authority as far as it will go?

Montesquieu is trying to convey the dangers of absolute power being vested on one body without checks and balances. He advocated separation of powers as a cure to the problem of one body becoming too powerful to the extent of threatening the freedom of the people

Montesquieu was a french political philosopher that is mostly known for his theory of the separation of powers.

Until that moment, most of the countries lived in a monarchy which the king held all the powers - legislative, executive and judicial - to himself. Because of it, Montesquieu said this phrase: "every man invested with power is apt to abuse it and carry his authority as far as it will go".

Knowing that power corrupts and that kings often abuse it, Montesquieu created the theory of separation of powers, which consisted of the separation of three branches of the government, which would prevent the abuse of power and would make possible for one branch to avoid the abuse of power of another branch.