Nan-in, a Japanese Zen master, received a visit from a university professor who wanted to learn about Zen. To demonstrate his teaching method, Nan-in served tea to the professor. He poured the professor's cup full, and kept pouring until the tea overflowed. The professor watched until he could no longer restrain himself, saying "It is overfull, no more will go in!" Nan-in responded "Like this cup, you are full of your own opinions and cannot learn Zen until you first empty your cup."