Bull markets are born on pessimism, grow on skepticism, mature on optimism and die on euphoria. 

- John Templeton

Great quote and everyone will likely agree with it, but what does it mean as a predictive tool. A quant is always looking for some indicator or number that can be used to describe the state of the market. How do we measure these terms: pessimism, skepticism, optimism, and euphoria?  We could use survey information, but it depends on the question. We are forced to find proxies and those are not easy to create.  Quotes are like candy - taste good but empty calories.