WOW! -- The Cup is Full

Nan-in, a Japanese Zen master during the Meiji era (1868-1912), received a university professor who came to inquire about Zen.

Obviously wanting to impress the master on his knowledge on zen he kept on lecturing for an hour. The master listened intently.

The master then invited the professor to tea. Nan-in served tea. He poured his visitor's cup full, and then kept on pouring.

The professor watched the overflow until he no longer could restrain himself. "It is overfull. No more will go in!"

 

What did the master mean?