Keynes on the Power of Ideas

I'm surprised to be favorably quoting Keynes, but I ran across this quote in Sowell's "on Classical Economics" and found it to be both true, and of a much wider application than just economics and politics (philosophy being the most important realm of its application).

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.