大般涅槃經義記 5.163

5.163

下廣釋之，解初門中，文殊先問，「若以真實為實諦者」牒上初門，下對佛等而為請門，若是真法即是如來虛空佛性，若即如來虛空佛性，如是三法便無差別。此如來者是其真實法身如來，如來是果，佛性是因。若論虛空，非因非果，彼前實諦據佛以論畢竟清淨，於此體上更無生死妄染可得，故即如來。就凡論之，猶為生死妄想所覆，與後顯時淨德為本，故名佛性。就實論實，實則平等，離一切性，故曰虛空。良以此三皆即實諦，故無差別。

Below, extensively explaining it: in expounding the first gate, Mañjuśrī first asks: 'If the true and real is taken as the real truth...' reiterating the first gate above, and below presents an inquiry with respect to the Buddha and so forth: If it is the true dharma, it is identical to the Tathāgata, space, and the buddha-nature; if it is identical to the Tathāgata, space, and the buddha-nature, then such three dharmas have no distinction. This Tathāgata is the true dharmakāya Tathāgata; the Tathāgata is the fruition, and the buddha-nature is the cause. If one discusses space, it is neither cause nor fruition. Discussing that prior real truth with respect to the Buddha, it is ultimately pure; upon this substance, no deluded defilement of saṃsāra can be found; therefore it is identical to the Tathāgata. Discussing it with respect to ordinary beings, it is still covered by the deluded thoughts of saṃsāra, yet serves as the root for the pure virtues when manifested later; therefore it is called buddha-nature. Discussing the real with respect to the real, the real is equal and free from all natures; therefore it is called space. Because these three are all truly identical to the real truth, there is no distinction.