Closing your mind means that you no longer perceive. The mind can not function without input or data. By inference closing your mind means that some perceptions, observations, or memory is discounted rather than used in making judgments. This means that you will be wrong more often that the mind that is open.
The progress made by mankind in the recent centuries is a result of testing ideas scientifically rather than relying on historical thinking. To close your mind is to preclude further progress.