摩訶止觀 5.131

5.131

今則非毒、非偽，故名為「真」；非空邊、非有邊，故名為「正」。如鳥飛空，終不住空；雖不住空，跡不可尋。雖空而度，雖度而空，是故名「誓與虛空共鬪」，故名「真正發菩提心」，即此意也。

Now, it is neither poisonous nor counterfeit, and is therefore called 'genuine'; it leans neither toward the extreme of emptiness nor toward the extreme of existence, and is therefore called 'correct'. It is like a bird flying in the sky, never dwelling in the sky; yet although it does not dwell in the sky, its tracks cannot be found. Although it is empty, one crosses them over; although one crosses them over, it is empty. Therefore, it is called 'vowing to contend with empty space', and is hence called 'arousing the genuine and correct aspiration for bodhi'—this is precisely the meaning.