Montesquieu advocated the separation of executive, legislative and judicial powers.
Montesquieu is among the greatest philosophers of liberalism, but his is what Shklar has called "a liberalism of fear" (Shklar, Montesquieu, p. 89). According to Montesquieu, political liberty is "a tranquillity of mind arising from the opinion each person has of his safety" (SL 11.6).
If it is to provide its citizens with the greatest possible liberty, a government must have certain features. First, since "constant experience shows us that every man invested with power is apt to abuse it ... it is necessary from the very nature of things that power should be a check to power" (SL 11.4).
This is achieved through the separation of the executive, legislative, and judicial powers of government. If different persons or bodies exercise these powers, then each can check the others if they try to abuse their powers.
But if one person or body holds several or all of these powers, then nothing prevents that person or body from acting tyrannically; and the people will have no confidence in their own security.
For more info on Montesquieu and the separation of powers, go to Stanford.edu and Libertyfund.org.