Select the best answer.

Which principle was James Madison describing when he wrote the following in The Federalist No. 51: “If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: you must first enable the government to control the governed; and in the next place oblige it to control itself.”?

representative democracy
popular sovereignty
separation of powers and a system of checks and balances
judicial review