This is one of the worst and often spoken phrases known to humanity. "I don't know", is a way to innocently maintain ignorance. It prohibits people from searching for answers, and when people remain in ignorance, bad things happen. Violence, war, economical breakdown...
I'm not saying this is the root of all evil, but it is a symptom in the great disease. I'm not pointing out the literal meaning of the words as much as I am the will to remain ignorant that resides behind the words.

Interesting comments there Catalyst. When I use that phrase, I am coming from a place of honest ignorance, as in I literally don't know the answer, and not from a place of willful ignorance, as in I never want to know the answer. In fact, it has been my observation that willful ignorance hides most often behind complete certainty, which is the exact opposite of the "I don't know" or beginner's mind.