大般涅槃經義記 10.67

10.67

自下第二明非內外如似虛空。「虛空無故非內非外」明喻似法。「佛性常故非內非外」辨法同喻。

From here below is the second point: clarifying that being neither internal nor external is like space. "Because space is non-existent, it is neither internal nor external" clarifies the metaphor resembling the phenomenon. "Because the buddha-nature is eternal, it is neither internal nor external" distinguishes the phenomenon as identical with the metaphor.