井の中の蛙、大海を知らず (いのなかのかわず、たいかいをしらず): the frog in the well knows nothing of the great ocean
井の中の蛙、大海を知らず
the frog in the well knows nothing of the great ocean
For someone confidently judging a world they have never seen. Often said about oneself, ruefully, after discovering how big a field really is.
The same saying in Chinese: 井底之蛙