妙法蓮華經玄義 8.115

8.115

二、空門者，即是彼教析正因緣，無明老死，苦集二諦，三假浮虛，破假實，悉入空平等，發真無漏，因空見真，空即第一義之門也。故須菩提在石室，觀生滅無常入空。因空得道，見佛法身，恐此是《成實論》之所申也。

Second, the gate of emptiness: this is analyzing direct causes and conditions in that teaching—ignorance, aging-and-death, the two truths of suffering and arising, the three designations being insubstantial and empty; breaking through provisional and real, all enter into the equality of emptiness, giving rise to the true untainted [wisdom]. Seeing the real through emptiness, emptiness is the gate of supreme meaning. Thus Subhūti, in his stone chamber, contemplated arising-and-perishing and impermanence, entering emptiness. Attaining the path through emptiness, he saw the Buddha's dharma-kāya; this is presumably what is expounded in the Tattvasiddhi-śāstra.