What was the rationale behind the system of separation of powers expressed within the US Constitution?

Answer 1

It all had to do with the English system of government against which the colonists rebelled. The English "mother country" relied on Aristotle's philosophical system of three types of government based upon the three types of people: a monarchy of the one, a aristocracy of the few, and a democracy of the many. Under the classical system, a monarchy that was corrupt and had lost its "virtue" (common welfare) would degenerate into a tyranny, an aristocracy that was corrupt would degenerate into an oligarchy, and a democracy that was corrupt would degenerate into an anarchy. The English tried to prevent degeneration by creating a government based upon the "three-legged stool" analogy: an effective government was one in which the monarchy (the King), the aristocracy (the House of Lords), and the democracy (the House of Commons) watched each other so that all the types of people would watch for a prevent degeneration in the others. This watchfulness is sometimes referred to as "vigilance" by the English revolution writers of the seventeenth century.

The House of Commons eventually evolved into a legislative body, the House of Lords served as a "wise old men" judicial function, and the King was to fulfill a mostly executive role. Therefore, the English had a governmental system that provided for checks and balances and a type of separation of powers. (There were, of course, many disagreements over the centuries about the functions of the three types of government, but that's another question.)

When the American colonists rebelled in 1776, they initially established a type of government that was based solely on the many. The Articles of Confederation, in effect from the Revolution until the adoption of the Constitution, was a national government in which each of the states were equally represented. Under the Articles, all legislative, executive, and judicial functions were performed by the Confederation Congress. And, by the way, the Articles could only be amended if all of the thirteen states agreed to change them. The American revolutionaries did not initially think that there was a need for the vigilance provided by the three-legged stool model because, being independent republican landholders, they were more virtuous than the English. There was, also, a lack of aristocrats in the colonies in the monarchy was only represented in the form of colonial governors. However, government function under the Articles of Confederation proved to be impossible; there were, indeed, some people in government who were not totally virtuous and the whole system appeared to be spiraling toward anarchy.

During the Constitutional Convention of 1787, the founding fathers sought a government that would provide checks and balances and the separation of powers but that did not rely on bodies that represented different types of people. They did not want to establish either an aristocracy or a monarchy. The Constitution is an amazing document in that it accomplishes this task; it provides for separation of powers between the different houses of congress, the presidency, and the judiciary. However, it does not rely upon simple virtue to accomplish a system of checks and balances. Each of the branches has its sovereignly only in the many. In theory, this type of democracy, will not degenerate into anarchy.