The Separation of Powers: Controlling the Threat of Factions.
In The Federalist 51, Madison sets out to explain the necessity of the constitutional setup of the government in preserving liberty. Since Madison is most concerned with the development of a faction, whether in government or among the people, which would threaten the liberty on which the United States is based, he advocates a system which would hopefully eliminate that threat. Madison finds that a government can only be safely powerful if the power is divided up enough that any one part does not have the ability to expand. This separation of powers must be true among the different sections of the government, as well as among the people. Liberty must be preserved and Madison finds that the setup of the government and the delegation of powers is the only way to achieve it. .
Federalist 51 clearly states that men are not "angels" who would be capable of governing themselves without an official centralized government and that it is the imperfection of human nature that necessitate government. On the other hand, giving power to any specified group creates the possibility that that power will be abused. Madison sees government as the "greatest of all reflections on human nature" and since human nature is not flawless, the government must take precautions by setting up external and internal controls. However, since it is universally agreed that men are creatures of ambition and specifically not angels, then it must be impossible to fully account for all possible ways that a powerful faction may develop and therefore impossible to eliminate the threat of a tyranny altogether. However, the government setup by the Constitution has many elements which aid in the control of this threat and which Madison sees as the most effective way to minimize the threat. .
In Madison's ideal world, the people would elect all divisions of the government into office so that the will of the people is theoretically always accounted for.