People who are a hundred percent sure about everything usually stopped reading a long time ago.

Certainty across every subject is a sign of a closed input rather than a full one. The more you actually learn about something, the more of its complications come into view, which is why real expertise usually arrives with qualifications attached. Total confidence is cheap to maintain, because it requires no new information. It is comfortable, and it is a form of standing still.

When you meet someone sure about everything, ask what would change their mind. Then ask yourself the same about your firmest opinion. If neither of you can answer, that is the finding.