The usefulness of the cup is in its emptiness

A learned man once went to a Zen teacher to inquire about Zen. As the Zen teacher explained, the learned man would frequently interrupt him with remarks like, "Oh, yes, we have that too..."

Finally, the Zen teacher stopped talking and began to serve tea to the learned man. However, he kept on pouring, and the tea-cup overflowed.

"Enough!" the learned man once more interrupted. "No more can go into the cup!"

"Indeed, I see," answered the Zen teacher. "If you do not first empty your cup, how can you taste my cup of tea?"