Final answer:
The main purpose of this excerpt is to outline the system of checks and balances among the three branches of government.
Step-by-step explanation:
The main purpose of this excerpt from The Federalist No. 51 is to outline the system of checks and balances among the three branches of government. It explains that the necessary partition of power among the several departments cannot rely solely on exterior provisions, and must be supplemented by the interior structure of the government in order to keep each branch in its proper place.
The best example that illustrates this purpose is the system of checks and balances among the three branches, where the Supreme Court has the power to declare a law unconstitutional, providing a check on the legislative branch.