肇論疏 2.23

2.23

Part four clarifies the essence. In this text there are three sections: first, directly clarifying the essence; next, explaining it; and last, citing two sūtras to establish it. This is the first.
Regarding "its": This refers to sacred wisdom. "Thing" refers to the nature of this sacred wisdom as an entity; this is calling it a "thing" when it is not a thing. "Real yet not existing, empty yet not non-existing": Knowing that all phenomena are empty is called "real." Although it is called real, its essential nature is not an entity, so it is said to be "not existing." Its essential nature not being an entity is termed "empty"; having nothing it does not know, it is said to be "not non-existing." One may also say: because of non-knowing it is said to be "not existing," and because there is nothing it does not know it is said to be "not non-existing."
Regarding "Existing yet impossible to discuss—can it be only sacred wisdom?": Because this wisdom is not non-existent, it is said to "exist." Because its characteristics cannot be discussed as definitively existing or definitively non-existing, it is said to be "impossible to discuss." Only sacred wisdom alone is like this, hence it says "only."