Although Yaakov and Eisav were twins and grew up in the same house, there were vast differences between their personalities as the Torah relates. "And the lads grew up, and Eisav became a man who knows how to trap, a man of the field; while Yaakov was a simple man who sat in the tents" (Bereishis 25, 27).
Rashi explains that Eisav's "knowing how to trap" refers to his ability to deceive. Eisav would ask his father seemingly spiritual questions thereby communicating a level of righteousness that he did not possess. In fact, he was an idol worshipper. In contrast, Yaakov was a "simple" man, one whose speech mirrored the way he felt in his heart. Rashi continues, "A person who does not use his cleverness to deceive is called simple."
Rav Wolbe (Shiurei Chumash) elaborates on Rashi's description of "a simple man." Yaakov was also extremely clever. However, he did not use his cleverness to deceive others. Moreover, he did not use his cleverness to deceive himself! When a person has negios (personal biases) he often subconsciously uses his cleverness to trick himself. Such a person could come up with a hundred reasons that justify his behavior, and not realize that each and every one of them was borne out of his prejudiced way of thinking.
Rav Wolbe would relate that the Chazon Ish once commented that it is possible for a great man to commit a sin and still be considered great. The fact that he might have failed to overcome his yetzer hara one time, does not detract from his being a great person the rest of the time. However, one who has negios cannot be considered a great man, since such a person is blinded by his negios twenty four hours a day. When exactly can such a person be considered a great man?
Many of our decisions in life are based on our personal biases. Often it doesn't make a real difference what we decide in the long run. Yet, in many circumstances, and especially in the spiritual arena, our decisions could have long lasting and serious ramifications. How can we be sure that we are not subconsciously deceiving ourselves? Ask a great person. One of the amazing things about our Torah leaders is their lack of negios. They have the ability to rise above all personal biases and scrutinize things objectively. It is for good reason that Chazal tell us (Pirkei Avos), "Make for yourself a Rav!"