大般涅槃經義記 6.17

6.17

上來八空以無破有，「第一義空」以有破無，名前八空為第一義，一切諸法空而常有，故非定無，非定無故，無第一義自性可得，是故名為第二義空。

The above eight emptinesses use non-existence to refute existence; 'ultimate emptiness' uses existence to refute non-existence. It designates the preceding eight emptinesses as the ultimate truth: all dharmas are empty yet constantly existent, hence they are not definitively non-existent; because they are not definitively non-existent, there is no self-nature of the ultimate truth that can be apprehended; therefore it is called ultimate emptiness.