Why does John Maynard Keynes end "The General Theory" with "Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist."?

Keynes' statement is part of a long tradition, insisting that philosophy influences us, even when we are ignorant of the philosophy.  It's easy to write this off as wishful thinking by philosophers (and economists). But read Betty Friedan's The Feminine Mystique to see how abstract, academic ideas spread into other academic areas, into popular culture, and into popular magazines.  Freudian ideas of sex roles became part of Margaret Mead's anthropology, part of college courses, part of advice in popular magazines, and finally part of how everyone 'knew' women and men should behave.

Sometimes they are the slaves of long dead Noble prize winners and assumptions of a gaussian distribution of outcomes... Nassim Taleb has done well pointing this out. http://en.wikipedia.org/wiki/The_Black_Swan_(Taleb_book) Another example:  I worked for a pretty good buisness unit manager, who was determined to have the #1 or #2 market share for every line of business.  He said that this was because the 1 & 2 players usually made the most money. (which is generally true).  He attributed this wisdom to Jack Welch... ( ;-) ) The market share / learning curve / profitability connection was formally developed by  Bruce Henderson at Boston Consulting Group (BCG), and popularized by BCG and the people who left BCG and formed Bain. Those ideas had been published about 20 years before, in the 1970s. You would thought these ideas came down on stone tablets with Moses, instead  of being theories based on a limited set of observations. If we wait about 5 more years, I expect we will see even John Boyd's strategic insights showing up on Corporate Powerpoints.

I don't know what Keynes really meant, but my guess is he was saying that ideas developed by economists do affect the framework of practical decision-making, albeit with a time lag. This may be through directly influencing their thinking, or indirectly, by affecting the option set within which they have to make choices. The mechanism of this propagation may be so indirect that the practical folk don't have a clue about it. One might interpret this as a clarion call to economists to understand the long term impact their ideas might have, and hence be more careful about the theories they spread. Anti-Keynesians might note the irony of the quote: practical people in politics, business, and the media in the 21st century continue to be deeply influenced by Keynesianism, the ideas of the defunct economist Keynes.