The theory of separation of powers was first propounded by the French thinker Montesquieu (in his book The Spirit of Laws). In Chapter XI of his book The Spirit of Laws Montesquieu writes:
When the legislative and executive powers are united in the same person, or in the same body of Magistrates, there can be no liberty; because apprehensions may arise, lest the same monarch or senate should enact tyrannical laws, to execute them in a tyrannical manner.
Again, there is no liberty, if the judicial power be not separated from the legislative and 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 of trying the causes of individuals.
Montesquieu’s theory of separation of powers, outlined in “The Spirit of Laws,” emphasizes that liberty is at risk if legislative, executive, and judicial powers are not distinct. Combining these powers can lead to tyranny and oppression, as it makes arbitrary control by the same entity possible.