Like this Cup, You are also too Full

In the early 20th century, Zen master Nan-in received a university professor who came to ask about Zen. But instead he only talked on and on about his own ideas. Nan-in served tea. He poured his visitor’s cup full, and then, while the man continued to speak, Nan-in kept on pouring the tea. 

The professor watched the overflow until he could no longer restrain himself. “You fool! It is overfull. No more will go in!” Nan-in replied, “Like this cup, you are also too full of your own opinions and speculations. How can I show you Zen unless you first empty your mind?”