Most Overrated Idea:

Progress. It is the notion that not only things but institutions and human nature get better and better and more and more rational as time goes on. This notion of linear or evolutionary progress is of course antiquated and has been disproved not only by history but by some “discoveries” in the natural sciences. It goes contrary not only to historical experience and reason but also to the appreciation of human nature and of the limits of human knowledge and character that almost all the Founders of the Republic understood and that is also an element in the essence of the Constitution. The unthinking American veneration of progress has been noted by most intelligent foreign observers of the United States. For one, Tocqueville in one of his letters assailed the very adjective progressif as, barbaric and senseless.

Most Underrated Idea:

The least appreciated element (it is an element, rather than an idea) is the understanding of the British (by which I mean English, Welsh, Scottish, and Irish) origins of American liberties. The freedoms that the Founders and that most of the American people wished to secure and protect were not abstract freedoms but certain British guarantees of freedoms in the eighteenth century to which they or their ancestors had been accustomed. Had the American colonies been settled by Frenchmen or Spaniards in the eighteenth century, American independence would have come in any event, sooner or later. But the entire political, legal, and judicial structure of a new American state would have been fundamentally different.