Dedicating your mind to a single thought

Priest master Shi Yan Hui was teaching me in the garden.
“You must purify your thoughts. When you think of rain, you must only see rain.
Not the trees it lands on or the clouds it comes from,” he instructed.
I was sitting in lotus position, and I nodded.
“I bet you have never had a pure thought in your life, have you!” he began to rant.
“How can you expect to master Wudan, if you cannot give your whole mind to anything?”
He stood up and pulled a thin flexible branch from underneath his robe.
“I will show you. . .