As has often been repeated by this Court, the doctrine of separation of powers is the very wellspring from which the Court draws its legitimacy. Former Chief Justice Reynato S. Puno has traced its origin and rationale as inhering in the republican system of government:
The principle of separation of powers prevents the concentration of legislative, executive, and judicial powers to a single branch of government by deftly allocating their exercise to the three branches of government…
In his famed treatise, The Spirit of the Laws, Montesquieu authoritatively analyzed the nature of executive, legislative and judicial powers and with a formidable foresight counselled that any combination of these powers would create a system with an inherent tendency towards tyrannical actions…
Again, there is no liberty, if the judiciary power be not separated from the legislative and the executive. Were it joined with the legislative, the life and liberty of the subject would be exposed to arbitrary control; for the judge would be then the legislator. Were it joined to the executive power, the judge might behave with violence and oppression.
There would be an end of everything, were the same man or the same body, whether of the nobles or of the people, to exercise those three powers, that of enacting laws, that of executing the public resolutions, and that of trying the causes of individuals. 
The principle of separation of powers is explained by the Court in the leading case of Angara v. Electoral Commission:37
The separation of powers is a fundamental principle in our system of government. It obtains not through express provision but by actual division in our Constitution. Each department of the government has exclusive cognizance of matters within its jurisdiction, and is supreme within its own sphere. But it does not follow from the fact that the three powers are to be kept separate and distinct that the Constitution intended them to be absolutely unrestrained and independent of each other. The Constitution has provided for an elaborate system of checks and balances to secure coordination in the workings of the various departments of the government. x x x And the judiciary in turn, with the Supreme Court as the final arbiter, effectively checks the other department in its exercise of its power to determine the law, and hence to declare executive and legislative acts void if violative of the Constitution.38