大般涅槃經義記 8.139

8.139

第四段中，「中道名性」，言少不足，若具，應言見空不空，乃至見我及與無我，是名中道，如是中道名為佛性。翻前可知，故不具論。由見空等知法非有、見不空等知法非無，故名為中，說此中道為佛性矣。

In the fourth section, the phrase "the middle way is called the nature" is brief and incomplete. If it were fully stated, it should say: "Seeing emptiness and non-emptiness, and likewise seeing the self and the non-self—this is called the middle way; such a middle way is called the buddha-nature." This can be understood by reversing what came before, so it is not discussed in full. Because through seeing emptiness and so forth one knows dharmas are not existent, and through seeing non-emptiness and so forth one knows dharmas are not non-existent, it is therefore called "middle"; this middle way is stated to be the buddha-nature.