James Madison on Human Nature and Government
"The Federalist. No. LI.", Alexander Hamilton or James Madison, orig. Friday, February 8, 1788." in The Federalist: A Commentary on the Constitution of the United States (G.P. Putnam's Sons: 1902), pp. 323-4.
What is government itself but the greatest of all reflections on human nature? If men were angels, no government would be necessary. If angels were to govern, 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. A dependence on the people is, no doubt, the primary control on the governement; but experience has taught mankind the necessity of auxiliary precautions.
This policy of supplying, by opposite and rival interests, the defect of better motives, might be traced through the whole system of human affairs, private as well as public. We see it particularly displayed in all the subordinate distributions of power, where the constant aim is to divide and arrange the several offices in such a manner as that each may be a check on the other — that the private interest of every individual may be a sentinel over the public rights. These inventions of prudence cannot be less requisite in the distribution of the supreme powers of the State.