大般涅槃經集解 56.36

56.36

案僧亮曰：明佛性是常，眾生用之，如身中虛空而得生長，是空之用。空非三世，以眾生有虛空界，故有空用也。佛性亦非三世，而是眾生有也。以解惑相礙，相礙故惑滅，惑滅乃用，故說為未有也。

Sengliang comments: This clarifies that the buddhadhātu is permanent. Sentient beings utilize it just as the space within the body enables growth; this is the functioning of space. Space does not belong to the three periods of time; because sentient beings have the element of space, there is the functioning of space. The buddhadhātu also does not belong to the three periods of time, yet sentient beings possess it. Because understanding and delusion obstruct one another, and due to this obstruction, delusion must be extinguished—only when delusion is extinguished is it functioning—it is therefore described as not yet existing.