Dongshan's Just This Is It

KOAN:

When Dongshan was ready to leave his teacher, Yunyan, Dongshan asked, “Later on, if someone asks me if I can depict your reality, or your teaching, how shall I reply?”
Yunyan paused, and then said, “Just this is it.”
When he heard that, Dongshan sank into thought. And Yunyan said, “You are in charge of this great matter. You must be most thoroughgoing.”
Dongshan left Yunyan and was still perplexed; he didn’t quite get it. As he proceeded he was wading across a stream, and seeing his reflection in the water, he had some understanding. He looked down in the stream and saw something, and then he wrote this poem:

‘Just don’t seek from others or you’ll be far estranged from yourself. Now I go on alone, but everywhere I meet it. It now is me; I now am not it. One must understand in this way to merge with suchness.’