This is not wholly true. Sometimes an original datum can remain uninspected, even in a bright person, such that they cannot receive or operate on new, correct data. There is a technique to handle these false datums by helping the individual expose them to view, thus relinquishing their hold on his thinking.
At other times a datum can be given to a person when he is in a state of fear, grief or apathy such that the datum has the power of a hypnotic suggestion.