Nan-in, a Japanese master met a university professor who came to inquire about Zen.
Nan-in served tea. He poured until cup full, and then kept on pouring.
The professor kept watching the overflow until he no longer could restrain himself. “It is overfull. No more will go in!”
“Like this cup,” Nan-in said, “you are full of your own opinions and speculations. How can I show you Zen unless you first empty your cup?”