大乘義章 1.89

1.89

第一釋名。入不二門，如《維摩》說。言不二者，無異之謂，即是經中一實義也。一實之理，妙寂離相，如如平等，亡於彼此，故云不二。

First, explaining the name. "Entering the gate of non-duality" is as explained in the Vimalakīrti[-nirdeśa]. Speaking of "non-dual," it means without difference; that is, the meaning of the one reality in the sūtras. The principle of the one reality is wondrously tranquil and free from characteristics, equal in suchness, with "this" and "that" obliterated; therefore it is said to be non-dual.