You also have to have the knowledge to enable you to make a very general estimate about the value of the underlying business. But you do not cut it close. That is what Ben Graham meant by having a margin of safety. You don't try and buy businesses worth $83 million for $80 million. You leave yourself an enormous margin. When you build a bridge, you insist it can carry thirty thousand pounds, but you only drive ten-thousand-pound trucks across it. And that same principle works in investing.

—Warren Buffett