Quotations

"I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions whch they have delighted in explaining to colleagues, which they have proudly tauaght to others, and which they have woven, thread by thread, into the fabric of their lives."
       Leo Tolstoy

Such persons as Mr. Tolstoy describes have stopped learning, haven't they?  Learning is being ready to question what you know when what you know is challenged by a new idea.