William Bernstein just wrote a book called "Deep Risk: How History Informs Portfolio Design". I haven't read the book but in the January issue of Financial Planning magazine there is an article talking through the concepts. Bernstein defines deep risk a negative real (inflation adjusted) return over a 30 year period. This can be caused by four things---inflation, deflation, confiscation, and devastation. According to the article the only area that an investor should have constant protection against is inflation, the other risks are too low in probability and/or have a high cost of insuring against them. This is a better idea than the permanent portfolio concept of having fixed allocations to things like gold, silver, treasuries, real estate, stocks, etc. but it is still flawed. Bernstein argues that to protect against inflation investors should own global equities, commodity producing stocks, gold, and tips. These assets may protect against inflation but they can also do poorly in other environments, like gold did in 2013.
We know that markets will go up and down, interest rates will go up and down, inflation will do up and down, commodity prices will go up and down, etc. We just don't know when. Having fixed allocations to protect against any of these risks is like using a sledgehammer to kill a bug--it may work but it is overkill and can cause collateral damage. None of these markets events happens overnight. If you keep in harmony with market trends you will automatically adjust your portfolio to be responsive to anything that could and will happen.