大乘起信論裂網疏 2.27

2.27

此申明真實空者，但表真如體上本無染妄，故以空字遮其妄有，非指此空以為真如體也。現前介爾心性，從本已來覓之了不可得，如何得與染法相應？如何得有差別法相？何處可容虛妄分別？是故有無四相、一異四相無不皆空，乃至一切妄分別心，總不能觸證此心性也。然雖云妄分別心所不能觸，只此妄分別心便自覓之了不可得，乃至一切染法本不可得，一切差別本不可得，豈俟以空遣之然後空耶！若遣妄存空，空仍是妄。今言空者，但遮妄念以明本空，非指此空為真如也。

This clarifies that "genuinely empty" merely expresses that on the essence of tathatā there is originally no defiled delusion, and so the word "empty" is used to refute its deluded existence; it does not point to this emptiness as the essence of tathatā. As for the momentary mind-nature present before us, seeking it from the beginningless past it is completely unattainable; how could it associate with defiled dharmas? How could it possess differentiated marks of phenomena? Where could there be room for false discrimination? Therefore, the four marks of existence and non-existence, and the four marks of oneness and difference, are all empty, up to all falsely discriminative minds being entirely incapable of touching and realizing this mind-nature. However, although it is said that the falsely discriminative mind cannot touch it, this falsely discriminative mind itself is completely unattainable when sought, up to all defiled dharmas being originally unattainable and all differentiations being originally unattainable; how could one wait for emptiness to eliminate them before they become empty! If one eliminates delusion and preserves emptiness, emptiness is still delusion. Now, in speaking of "empty", it merely refutes deluded thoughts to make clear original emptiness; it does not point to this emptiness as tathatā.