妙法蓮華經玄義 8.124

8.124

諸法既如幻，故名為有。幻不可得，故名為空。如鏡中像，見而不可見，不可見而見，是亦空亦有門。

Since all dharmas are like an illusion, they are termed existent. Because the illusion cannot be found, they are termed empty. Like an image in a mirror, seen yet invisible, invisible yet seen; this is the gate of both emptiness and existence.