大方廣佛華嚴經隨疏演義鈔 37.36

37.36

疏「既如虛空，何有無為之相」者，我言無盡者，體無可盡，故如虛空。非謂有於一物，若高山之出雲，用之無盡也。故云智者說無盡，此亦無所說。由盡即無盡，即非盡非無常矣。無盡既如虛空，則非無盡非是常矣。是為雙非常無常。雙離盡不盡，亦雙非為無為。故《中論》云「若有有為法，則有無為法。既無有為法，何得有無為？」

As for the commentary's phrase “since it is like empty space, how could there be a characteristic of the unconditioned?”: when I speak of inexhaustibility, its essence has nothing that can be exhausted, and so it is like empty space. It does not mean that there is an entity, like clouds emerging from a high mountain, whose function is inexhaustible. Hence it says that the wise speak of inexhaustibility, yet here nothing is spoken of either. Because exhaustion is precisely inexhaustibility, it is neither exhaustion nor impermanence. Since inexhaustibility is like empty space, it is neither inexhaustibility nor permanence. This is the dual negation of permanence and impermanence. It doubly departs from exhaustion and non-exhaustion, and also doubly negates the conditioned and the unconditioned. Thus the Madhyamakaśāstra says: “If there were conditioned dharmas, then there would be unconditioned dharmas. Since there are no conditioned dharmas, how could there be the unconditioned?”