Walter Berns, "Natural Rights and Modern Constitutionalism," Natural Law, Natural Rights, and American Constitutionalism, a web resource of the Witherspoon Institute.
The idea of constitutionalism is as old as political science, and its features are best described and defended by political philosophers. Aristotle, for example, first addressed the question of the best form of government and, after weighing all the relevant factors, decided in favor of a “mixed regime,” specifically a mixture of oligarchy and democracy, or as he put it, a system of rule by the middle class. Such a regime, he said, would be free from factions and, for that reason, was likely to be stable and moderate. He gave it the name “polity”; this can be said to be his version of constitutional government.