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

In his book, "The Spirits of the Law", Montesquieu writes this pasage advocating for the separation of powers within the government.

He explains that political liberty can only exist when there is no abuse of power, and history has shown a repetitive abuse of power by the man who held it.

Therefore, Montesquieu calls for a system of checks and balances in which no man is compelled to do what the law does not oblidge him nor does he have to refrain from doing what it allows.