Political liberty of the individual citizen is that tranquility of spirit which possesses its own assurance; and to secure that liberty, it is essential that the government permit no citizen to fear another citizen. But when the legislative and executive powers are united in the same person, or in the same political group, there can be no liberty; because then the tendency will be for this group to inflate its powers through the enactment of tyrannical laws, which it will then execute in a tyrannical manner.
—Charles de Secondat, Baron de Montesquieu, De l’esprit des lois, bk. xi, ch. vi (1748), in Œuvres complètes de Montesquieu, vol. 2, p. 397 (Pléiade ed. 1951) (S.H. transl.)