The ideas of economists and political philosophers, both when they are right and when they are wrong, are more powerful than is commonly understood. Indeed the world is ruled by little else. Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist. Madmen in authority, who hear voices in the air, are distilling their frenzy from some academic scribbler of a few years back.

— John Maynard Keynes, The General Theory of Employment, Interest and Money (1935)

Over a hundred years ago, the German poet Heine warned the French not to underestimate the power of ideas: philosophical concepts nurtured in the stillness of a professor's study could destroy a civilization. He spoke of Kant's Critique of Pure Reason as the sword with which German deism had been decapitated, and described the works of Rousseau as the blood–stained weapon which, in the hands of Robespierre, had destroyed the old regime; and prophesied that the romantic faith of Fichte and Schelling would one day be turned, with terrible effect, by their fanatical German followers, against the liberal culture of the West. The facts have not wholly belied this prediction. (..) To neglect the field of political thought, because its unstable subject–matter, with its blurred edges, is not to be caught by the fixed concepts, abstract models and fine instruments suitable to logic or to linguistic analysis — to demand a unity of method in philosophy, and reject whatever the method cannot successfully manage — is merely to allow oneself to remain at the mercy of primitive and uncriticised political beliefs. It is only a very vulgar historical materialism that denies the power of ideas, and says that ideals are mere material interests in disguise.

— Isaiah Berlin, Two Concepts of Liberty (1958)