The most significant principles had to do with the checks and balances system of government. They divided the government into three branches. The judicial, executive and legislative branch all served to check and balance each other so that no one government has all of the power.
I think they included these principles to prevent any one person or family from having too much power and control. As mentioned above most of the checks and balances were put in place to prevent what happened in England from happening here.
I agree with brettd that much of the Constitution was written in reaction to the colonists' experience with the British monarchical system of government. In particular, the creation of a tripartite system of government, with three roughly equal branches, would theoretically prevent a tyrant from taking control of the country. The checks and balances imposed by the executive, judicial, and legislative branches all work together to ensure that authority is spread widely enough...
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