Warren Buffett has two mentors: his dad Howard Buffett, and Ben Graham. At the age of 74, Warren Buffett thinks that the best advice he ever got from his mentors was about independent thinking: you're neither right nor wrong because others agree with you, you are right because your facts and reasoning are right…
http://www.fortune.com/fortune/articles/0,15114,1034769,00.html
http://www.fortune.com/fortune/articles/0,15114,1034769,00.html