大乘起信論裂網疏 2.30

2.30

此申明真實不空者，但能了達妄念本空，即顯真心常恒不變，淨法圓滿，故以不空表之，不同妄念所計不空相也。若以妄念所計不空為真如相，則同餘宗所計離色心等有實常法名為真如，其謬甚矣。文中常恒是常德，不變是我德，淨法是淨德，圓體是樂德。四德不可思議，故唯離念者之所證也。初釋真如門竟。

This clarifies that "genuinely non-empty" means that if one can only realize that deluded thoughts are originally empty, the true mind is revealed to be eternal and immutable, replete with pure dharmas, and so it is expressed as non-empty; it is not the same as the mark of non-emptiness conceived by deluded thoughts. If one takes the non-emptiness conceived by deluded thoughts to be the mark of tathatā, that would be identical to the conception of other schools that apart from form, mind, and so forth there is a real, permanent dharma called tathatā—which is an extreme error. In the text, "eternal" is the virtue of permanence; "immutable" is the virtue of self; "pure dharmas" is the virtue of purity; "replete essence" is the virtue of bliss. The four virtues are inconceivable; hence it is realized only by those who are free from thoughts. The first part, explaining the aspect of tathatā, is concluded.