Graham introduced “Mr. Market” in The Intelligent Investor in 1949, the conceptual entity who is every stockholder’s counterpart and co-investor. He noted that Mr. Market is prone to periods of over-optimism and excessive pessimism in setting the prices at which he’ll trade. At Brandes, our philosophy, underpinned by 50 years of practical experience, is that in order to profit from the market’s excesses of optimism and pessimism, the key is to research the fundamental value of the business behind the stock. That information enables us to be “realists”—neither optimists, nor pessimists.     

Source: The Intelligent Investor: A Book of Practical Counsel—Revised Edition, Benjamin Graham. Updated with New Commentary by Jason Zweig (HarperCollins, 1973, 2003)