Succinctly, the fundamental idea presented by Montesquieu is to separate the power, functions and people of the three parts of any government in order to prevent abuse and concentration of power.
There exist tensions between the motives and procedures of the executive, legislature and judicial bodies of a government, and this is good for the stability and working of the government. Power separation requires that all these remain in a state of active relationship among them with no one being able to dominate the others resulting in a balanced see-saw. But still there are some political powers in the government which are exercised by more than one governmental body. All three bodies of the government exercise functions which should be separate in the strict sense of separation of power. The executive exercises legislative powers by dominating the Congress. The judiciary, subtly, creates law. Congress is able to discipline its members, without the involvement of the courts. So, the separation of powers between the judiciary and the Congress, in reality, is more of a sharing of powers than a separation and therefore, judiciary play an active part in rulemaking. Introducing the Bill of Rights, James Madison stated:.
"Independent tribunals of justice will consider themselves in a peculiar manner the guardians of those rights; they will be an impenetrable bulwark against every assumption of power in the legislative or executive; they will be naturally led to resist every encroachment upon rights expressly stipulated for in the Constitution by the Declaration of Rights.".
In the American Constitution, each branch has institutional incentive to check the others. Each branch is equipped with the tools to check the others" natural tendency to expand their power and influence. It is based on the Rule of Law, which contemplates that the government is of law and not of men. It requires law as the basis for government action, and subject government to the constraints of law and without the separation of legislation and execution of laws, the rule of law cannot function.