Following the second moon to seek the first moon is investigating Chan

If someone provokes me, I give him a whack with a stick—and this whack is the second moon, an echo in an empty valley. Rely on this to seek the true and original first moon, to seek the source of this second moon. How exhilarating that is, utterly refreshing and free! Why must you be like a blind cat bumping into a dead rat, sitting there in meditation all day long, sitting until there are no more delusive thoughts, enjoying a moment of tranquility, and then comparing it against the pure nature of the eighth consciousness, thinking: this tranquil state, with no thoughts and no delusions—isn't this my original mind, the eighth consciousness? Isn't this my Dharmakāya Buddha, the Tathāgata?

You fool! Who taught you to recognize the Tathāgata Buddha this way? If so, then when you have delusive thoughts, when you have thoughts arising, is your Tathāgata Buddha dead, not yet born? Later, when your meditative concentration fades and thoughts arise and delusions appear, does your Tathāgata Buddha then vanish and perish? If so, how could your Tathāgata Buddha be subject to arising and ceasing? How could it be subject to increase and decrease? How could it be subject to change? How could it exist one moment and not the next, requiring constant watching over it?

The fact is, this tranquil state of yours, without thoughts and delusions, is just the second moon, turned around from the first moon. Do not linger on the second moon; hurry and chase along its path to find the first moon. That first moon is intrinsically pure, pure and free of thoughts at all times, existing naturally without your needing to sit in meditation. That alone is your true mind, the Dharmakāya Tathāgata Buddha.

There are also others who mistakenly take the gap in the middle—where, with a snap, the prior thought has ceased and the subsequent thought has not yet arisen—taking this that is vividly solitary and luminous, distinct and clear, utterly lucid and plainly evident, to be their own original mind, the Lord Tathāgata Buddha, which is without thought or ideation, intrinsically pure, unborn and undying. If so, before you snapped and cut off the delusive thoughts, where was the Lord Buddha? Had He not yet been born? And when delusive thoughts appear a moment later, where has your Lord Buddha gone—extinguished? When there are no delusive thoughts, what is it that is so distinct and clear? What is it that is so plainly evident? Clear and evident about what, exactly?

The fact is, this gap in the middle—where the prior thought has ceased and the subsequent thought has not yet arisen—is only the second moon, conjured forth from the first moon. You must follow the state of vivid solitary luminosity to seek the original first moon behind it. This is Chan investigation (canchan). The true mind, the self-nature Tathāgata, has never been distinct-and-clear about your present state of tranquility without delusive thoughts, nor has He ever been distinct-and-clear about any worldly state of the five aggregates. He knows nothing at all—no seeing, no hearing, no feeling, no knowing; neither solitary nor vivid; neither in the middle nor at either end. Rather, it is the illusory dharma of arising, ceasing, increasing, and decreasing—the mind consciousness—that has delusive thoughts one moment and none the next, is clear and evident one moment and muddled and unclear the next, solitary and vivid one moment and clamorous the next. When you truly recognize the Buddha of your own mind, then no matter what you are doing—even killing and arson—the Dharmakāya Buddha of your own mind still exists and still functions wondrously. He has never been extinguished; at every instant there are corresponding functions and operations. He is not some vacant, dull-witted, do-nothing fool of a Buddha.

He does not merely flash before your eyes; He also quietly serves you from behind your back. He does not appear only when your mind is pure, free of delusions and afflictions; He also appears when you are being reckless and shameless, and also appears when you are wreaking havoc in Heaven, toppling hills and overturning mountains. He appears not only when you ascend to heaven, but also when you descend into hell. If you have truly discovered Him, then you will be able to discover Him everywhere; if you cannot discover Him elsewhere, then what you discovered here is not Him. When you truly know a person, you can recognize that person wherever he goes; if you have misrecognized a person, then in any other place you will be unable to recognize the real person.