Stick to your Circle of Competence

Charlie Munger once commented that "in order to disagree with somebody you must first understand their argument better than they do."

The most common way people lose money is to overestimate how clever they are and how much they know. Even after a fifty-year investing career, both Warren Buffett and Munger read over 500 pages a day and still limited themselves to a small circle of competence where they can explain the other side's argument better than they can.