A different time and place, but James Madison seems to address this question in Federalist #51.
The people of the new republic, the Anti-Federalists most vocally, were concerned that a strong government would devolve into an oppressive aristocracy or monarchy.
Madison argues that by creating three jealous branches - executive, judicial, and legislative - the government will guard itself. Not through a Socratic notion of virtue, but though self interest and greed. The legislature, designed as the most powerful branch will be divided again to prevent overreach into a senate and a house. Madison asserts:
"...The great security against a gradual concentration of the several powers in the same department, consists in giving to those who administer each department the necessary constitutional means and personal motives to resist encroachments of the others. The provision for defense must in this, as in all other cases, be made commensurate to the danger of attack. Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. It may be a reflection on human nature, that such devices should be necessary to control the abuses of government. But what is government itself, but the greatest of all reflections on human nature? If men were angels, no government would be necessary."
The last quote: "If men were angels, no government would be necessary." seems to be an attempt to answer that vexing question - if virtue isn't enough, what is? His answer seems to be a properly constructed government.
Establishing distinct branches of government, all with an eye towards their own power will, in theory, prevent audacious power grabs by another branch.
This theory is useful, but dangerous. By creating a system whereby each branch jealously protects their own power, while coveting more, gives incentive to the strongest branch to expand at the expense of the existing branches. In other words, the only time the guardian-over-guardian approach will be sufficient is when each branch is essentially equal in power and when each branch is incentivized to guard their power rather than accumulating more of it.