From Montesquieu, The Spirit of the Laws, vol. 1, trans. Thomas Nugent (London: J. Nourse, 1777), pp. 221-237, passim.
“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 power of judging be not separated from the legislative and executive powers. Were it joined with the legislative, the life and liberty of the subject would be exposed to arbitrary control, for the judge would then be the legislator. Were it joined to the executive power, the judge might behave with all the violence of an oppressor.
“There would be an end of every thing 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 that of judging the crimes or differences of individuals.”