The separation of powers was first introduced into political discussion during the English Civil Wars of the seventeenth century by the political party known as Levellers. The object was to insure that persons be judged by general and prospective rules. If the legislative authority should decide a particular case, it might be tempted through partiality or prejudice to improvise a special rule for the situation. So the separation of powers was intended to achieve that impartiality in government which Aristotle called "the rule of law."
The doctrine of checks and balances was also introduced into political discussion during the Civil Wars, and with the Stuart Restoration in 1660 it became the official description of the English constitution. King, Lords and Commons were in a condition of equilibrium. "Like three distinct powers in mechanics, they jointly impel the machine of government in a direction different from what either, acting by itself, would have done; but at the same time in a direction partaking of each, and formed out of all; a direction which constitutes the true line of the liberty and happiness of the community.' By the time Blackstone wrote these words, the tripartite division of legislative power had already yielded to what we call today the cabinet system.
Francis D. Wormuth,
Legislative Disqualifications as Bills of Attainder,
4 Vanderbilt Law Review
Available at: https://scholarship.law.vanderbilt.edu/vlr/vol4/iss3/9