What realm does the echo in the valley belong to?

When you shout loudly in a valley, that sound is the essential object. When the essential object encounters the air or the rock walls and is obstructed, the propagated sound becomes altered in quality, because the particles of the four great elements have changed. When we then hear the echo, the sound feels rather illusory and unreal, as if it came from a distant horizon, greatly different from the original human shout. The human shout is real, while the echo arises afterward as a transformation based on the shout—just like the second moon.

The echo is similar to the rebound of sound when it meets an obstruction, like a ping-pong ball rebounding off a paddle; the speed of the rebounding ping-pong ball is determined by the magnitude of the resistance it encounters. The volume and quality of the echo in an empty valley depend on the force of the air convection and the resistance of the rock walls. The more open and empty the valley, the more false, the more illusory, and the more unreal the echo becomes. The echo in a valley cannot be said to be real, nor can it be said to be false; it carries the nature of being both true and false, because it carries the substance of the essential object.

The second moon comes directly from the true moon in the sky, the first moon; without the first moon there would be no second moon. You cannot say the second moon is real, nor can you say the second moon is false. The second moon carries the nature of being both true and false, because it carries the substance of the essential object of the first moon—and it carries quite a lot of it, which is why it appears rather real. Even so, a person of wisdom, when pinching out the second moon, will not take it to be the true moon, the first moon. Likewise, one should not take the echo in an empty valley to be the real voice of a person; one can only follow the echo to seek the person's real voice, and thereby find the person. This is also the secret of investigating Chan.