In the movie World War Z a virus starts turning people into Zombies and they threaten to take over the world. The main character, played by Brad Pitt, is tasked with finding out what happened and how to cure it. On this mission he hears that Israel was the only country not overrun by the zombies because they built a massive wall before the outbreak. When he travels to Israel to find out how they predicted what was going to happen they tell him about the 10th man doctrine. This is a committee of 9 people and a 10th man. If all 9 people agree on something, it is the 10th man's responsibility to disagree, not matter how far fetched his findings might be, and prepare for that eventuality. When the Israelis first heard word of zombies all 9 people agreed that it was nothing, the 10th man then had to disagree and build a wall just in case there was a zombie apocalypse.
What does this have to do with investing? Everything. In investing the path of least resistance is to conform to what everyone else is thinking, that way if you are wrong everybody else is wrong at the same time. It is also easier to believe the most recent past will equal the future. So if we are in a bull market the consensus is that the bull market will continue, if we are in a bear market then the consensus is that the bear market will continue. The 10th man doctrine would force you to look beyond the consensus at other possible scenarios and prepare for them.
Since we have been in a bull market for 7 years one likely scenario is that it will keep going. If you listen to CNBC that is probably the consensus, so you must be prepared for it. However, what if it doesn't keep going? Other likely scenarios are a bear market or the market we have seen since August, with large corrections followed by large rallies without really going anywhere, continuing. Since you can't predict markets your portfolio has to be prepared for either of these eventualities as well.