Nan-in, a Japanese Zen master, received a visit from a university professor who wanted to know about zen. They sat and visited and talked for a while.
Nan-in then took out tea cups and proceeded to pour tea for his guest. He filled his guest's cup and continued to pour as the tea spilled over the sides. The professor, unable to restrain himself, cried : "Stop. The cup is full, no more will go in."
Nan-in replied: "Like this cup, you too are full. You are already full of your own judgments, speculations, ideas and opinions. How can you begin to learn until you empty your cup first?"
The lesson for us is that we cannot learn if we believe we already know everything. How often do you think you are open but in reality, your cup is already full? You must unlearn before you start to learn something new. Else it will obstruct your process of acquiring higher level of knowledge or skill.