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The quotation has been kicking around in the field of urban studies/environmentalism for decades. It was popularized by both Lewis Mumford and Rene Dubos. It was in a book by Albert Mayer in 1967. But Paul Valéry appears to have written it first. In any event, when applied to the stock market, it tells us simply that trends do not go on forever. Trends do, however, tend to last longer and go further than originally expected, and you can make money knowing this.