"What is Knowing? It seems that knowing is not the same to everyone. I know, you know yet there exists so many shades of differences. What is the reason for this?"
Question: Above you already said that nature on its own is “completely constant knowing.” Why is it necessary for the various Buddhas to open and indicate it?
Answer: That which is this word knowing is not indeed the knowing of evidence. It means to articulate the true nature that is not the same as the vast sky, a tree, or a rock. Therefore I said knowing. Neither is it like the conditional and objective discriminations of consciousness, nor like the shining essence of complete penetration of wisdom. It just is the one true suchness of nature autonomously and constantly knowing.
[For comparison here's Jeffrey Broughton's translation from Zongmi on Chan, p. 135:
Question: Above you have spoken of the “complete and constant Knowing that is intrinsically [pure from without beginning]’” Why should it be necessary for the buddhas to open it up and show it?
Answer: This Knowing is not the knowing of realization. My intention was to explain that the true nature is not identical to the sky or a tree or a stone, and, therefore, I said “Knowing.” [Knowing] is not like the consciousnesses that take sense objects as objective supports and discriminate. It is not like the wisdom that illuminates substance and comprehends. It is just that the nature of thusness is spontaneously constant Knowing. ]So Master Zongmi is making very important points about the knowing that is the common ground of beings.
"That which is this word knowing is not indeed the knowing of evidence."
"It means to articulate the true nature that is not the same as the vast sky, a tree, or a rock."
"Neither is it like the conditional and objective discriminations of consciousness,"
"nor like the shining essence of complete penetration of wisdom."
Here, Zongmi is making the Zen point that the true knowing is not even the penetrating wisdom of prajna. Prajna is the realm of the sages, bodhisattvas and Buddhas. Zongmi is saying that the true knowing is not this type of wisdom, but is the knowing that is the inherent knowing of both commoners and sages, sentient beings and Buddhas. If knowing was the purview of sages and Buddhas, then that knowinng would be a knowing of accomplishment and as a knowing of acomplishment it could not be the innate knowing of true suchenss.
"It just is the one true suchness of nature autonomously and constantly knowing."
This knowing is shared by everyone. This is the knowing indicated by the Zen phrase "ordinary mind is Buddha." But because we confuse this knowing of true suchness, this "ordinary mind," with ordinary perceptions, objective consciousness, knowing about things, penetrating wisdom, etc, Zongmi has given us the examples of these categories of mistaken notions of "ordinary mind" and "knowing."