Negative knowledge is the process of first looking at what does not work to eventually come to the realization of what does. Figuring out where others consistently go wrong is one of the best ways to ensure one's own success. This chapter enlists several mistakes that investors make on a regular basis. When buying individual stocks, people tend to invest in familiar companies. Investors worry only about what could go right with an investment and ignore what could go wrong. Investing is not only about understanding market history, portfolio construction, and mutual funds. None of that matters if investors do not understand how their natural human inclinations and cognitive biases can wreak havoc on their decision-making process. Furthermore, this chapter discusses the traits of a successful investor. The most important lessons about world's greatest investors have nothing to do with specific investments and everything to do with making good decisions and developing the correct temperament.