Every new moment is necessarily unique – we’ve never been “here” before. Probabilities and potential payoffs change based on the stage of the trend or cycle. For example, the current decline in stocks is no surprise, given the stage and magnitude of the prior trends. A few see evidence of the early stages of a bigger move, others believe it’s different this time. We’ll see how it all unfolds. I don’t have to know what’s going to happen next – I am absolutely certain of what I will do given different conditions.

To quote from fellow Tennessean, Sir John Templeton:

“The four most dangerous words in investing are, it’s different this time.”

– Sir John Templeton

source: http://www.templeton.org