You can control risk. You can't control return.

--- Vanguard founder Jack Bogle, in a BusinessWeek interview with Christopher Farrell, 27 October 2008

Context:

Christopher Farrell: On Wall Street the talk is about return, but both of you emphasize risk.

Bogle: You can control risk. You can't control return. That's up to the beneficence that stock and bond markets are generous enough to bestow on us. That's why we talk about diversification.

If you can't say it clearly, you don't understand it yourself

--- philosopher John Searle, via the virtual philosopher blog

According to brainyquote, the fuller version is

"Where questions of style and exposition are concerned I try to follow a simple maxim: if you can't say it clearly you don't understand it yourself. "