大方廣佛華嚴經隨疏演義鈔 75.45

75.45

疏「上之四門各雙存互奪以成四句」者，如第一常無常門云：一者常；二是無常；三合前，即成亦常亦無常，故云雙存；四約互奪，即成非常非無常，無常奪常故非常，常奪無常故非無常矣。謂正以體虛無實，即自性無常故。下三門例知也。

Regarding the commentary's "The above four gates each preserve both and mutually negate to form the four propositions": as in the first gate of permanence and impermanence, it says: first is permanent; second is impermanent; third, combining the preceding, forms both permanent and impermanent, hence called "preserving both"; fourth, in terms of mutual negation, it forms neither permanent nor impermanent—impermanence negates permanence, hence neither permanent; permanence negates impermanence, hence not impermanent. This means precisely because the essence is devoid of reality, it is by self-nature impermanent. The three gates below should be understood by analogy.