From "With Care", Henri J.M. Nouwen

There is a story ... About a university professor who came to a Zen master to ask him about Zen. Nan-in, the Zen master, served him tea.

"He poured his visitor's cup full, and then kept 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?"