He ends The General Theory (arguably his magnum opus) with the following:
Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist.
Here is a good discussion on the topic...
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http://www.quora.com/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)
I've always very much liked that quote. It is a full-frontal assault on anti-intellectualism.
The key word is "defunct." Keynes certainly did not consider himself a defunct economist. His point is that practical men (politicians, businessmen, whoever) who feel they are above the highfalutin ideas of economists are simply making decisions, whether they are aware of it or not, based on weak economic models that current ("non-defunct") economists would reject.
I find it similar to what some behavioral economists say about psychology in economics. It cannot be ignored. Pretending to ignore psychology in economics is just subconsciously endorsing a bad model of human psychology.
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.
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.
Topic: Keynes and The General Theory