Human beings can only know knowledge, that is, known and unknown, but can not know non knowledge, let alone how many non knowledge exist. An accomplished scientist not only knows known and unknown in his mind far more than ordinary people, but also keeps a high degree of vigilance to non knowledge at any time. Once the opportunity appears, he can break through the shackles of known in his mind and try his best to transform non knowledge into knowledge.
The known in the mind sometimes becomes an obstacle to human recognition of non-knowledge can be inferred from above.