"If men were angels, no government would be necessary. If angels were to govern men [people], neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men [people over people], 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.”
What is Madison making a case for in his quote from Federalist Paper 51? Review also the US Constitution in your textbook. Try to give one example from the constitution of what James Madison is talking about.
I believe Madison was saying that in order for government to govern the people they must also need to govern themselves first and this is through the "check and balances" between the three branches of government which are legislative, executive, and judicial. Each branch in itself has their own identity and power within themselves and they serve as a check and balance to the other two branches. In the Constitution Articles I, II and III state clearly what each branch can and cannot do and how they serve as checks on other two branches.
The importance of the separation of powers is the intent that men shall be ruled by law and not by men which I believe the above quote of Madison states. According pg 706 Appendix 8 in the textbook, "In affect competing personal interests in each branch will help to keep the powers of the three government branches separate and, in so doing, will help to guard the public interest."