Sanyasi @@'s Key Ideas from The Most Important Thing
by Howard Marks

Inefficiency is a necessary condition for superior investing. Attempting to outperform in a perfectly efficient market is like flipping a fair coin: the best you can hope for is fifty-fifty. For investors to get an edge, there have to be inefficiencies in the underlying process - imperfections, mispricings-to take advantage of.

The random walk hypothesis says a stock's price movements are of absolutely no help in predicting future movements. In other words, it's a random process, like tossing a coin. The hypothesis says, the fact that a stock's price has risen for the last ten days tells you nothing about what it will do tomorrow.