frog in a well; person with a narrow outlook
Literary idiom from the Chinese parable 井底之蛙 (frog at the bottom of a well). Used to describe someone who knows only their own small world and lacks broader perspective. Often appears in the set phrase 井蛙の見 (せいあのけん, 'the view of a frog in a well').
His opinion is nothing more than the view of a frog in a well.
A frog in a well knows nothing of the great ocean.