It may be too great temptation to human frailty, apt to grasp at power, for the same persons to have the power of making laws to also have in their hands the power to execute them, whereby they may exempt themselves.
Gist of Idea
The executive must not be the legislature, or they may exempt themselves from laws
John Locke (Second Treatise of Government , 143)
The main principles of modern constitutions are devised to avoid corruption. If people were incorruptible (yeah, right) the world would presumably be run very differently, and rather more efficiently, like a good family.
Locke,John: 'Two Treatises of Government' [Everyman 1988], p.190