Learning is harder than knowing. It requires effort both to grasp and to release previously held beliefs…
Nan-in, a Japanese master during the Meiji era, received a university professor who came to inquire about Zen. Nan-in served tea. He poured his visitor's cup full, and then kept on pouring. The professor watched the overflow until he could no longer 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?"
Zen Flesh, Zen Bones by Paul Reps and Nyogen Senzaki (Tuttle)