There is much "perception" behind indeed. In effect, we have - gasp - a social agreement of how we play the market game. As long as everyone agrees that a market is going up and that people will keep buying ever more pricey assets, everything will be going fine... except that there need to be ever more buyers. A collective illusion can go far, but not infinitely far.

But besides facts and perception of facts, we have numbers. They keep "surprising" WSJ analysts a few months in a row.