大乘起信論義疏 1.54

1.54

問曰：是互無耶？答：非是也。互無者就馬中無牛，故名馬空也。無他相者，就真法中，尋推生死，法相不可得故，故名為空。

Question: Is this mutual non-existence? Answer: It is not. Mutual non-existence refers to the absence of an ox in a horse, which is termed the horse being empty [of the ox]. "Without the characteristic of other" means that within real phenomena, when one investigates and seeks saṃsāra, the characteristics of phenomena cannot be obtained; therefore it is called "empty".