This Koan is called “Lingyun’s Peach Blossoms”

Lingyun was wandering in the mountains and became lost in his walking. He rounded a bend and saw peach blossoms on the other side of the valley. This sight awakened him and he wrote this poem:

For thirty years I searched for a master swordsman. 

How many times did the leaves fall and the branches break into bud?

But from the moment I saw the peach blossoms, I’ve had no doubts.

Centuries later the Japanese teacher Keizan responded with his own poem:

The village peach blossoms didn’t know

their own crimson

but still they freed Lingyun

from all his doubts.

Here are some comments on this Koan:

If we can truly master it, this koan can cut away all delusions. We have no more doubts, because we have experienced our Essential Nature. In Zen practice our doubts coexist with our direct experience. This tension is a prod to go deeper, to continuously refine our understanding.

Source: Mumonkan (Case 37)