仁王般若經疏 1.119

1.119

問：空與空空有何等異？答：空破五陰，空空破空。如服藥能破病，病破已藥亦應出，若藥不出即復是病。以空滅諸煩惱病，恐空復為患，是故以空捨空，故名空空也。

Question: What is the difference between emptiness and the emptiness of emptiness? Answer: Emptiness shatters the five aggregates; the emptiness of emptiness shatters emptiness. Just as taking medicine can cure disease, once the disease is cured the medicine should also be expelled; if the medicine is not expelled, it becomes a disease again. Using emptiness to eliminate the diseases of all defilements, fearing that emptiness may again become an affliction, one therefore abandons emptiness with emptiness; hence it is called the emptiness of emptiness.