For Mill, the acknowledgment of his or her own fallibility is part of what makes someone a serious thinker. Human knowledge progresses when people recognize that they may be wrong even on issues that seem certain to them. Wisdom involves openness to those who disagree with us. It is only when our ideas have been subjected to criticism and all objections considered--if necessary seeking these objections out--that we have any right to think of our judgement as better than another's.