首楞嚴義疏注經 10.133

10.133

此但現見行陰遷變，生中有滅故非有，滅中有生故非無，由是通悟一切皆是非有、非無，何啻於八。虛實即無有也，有不定有，實何曾實？無不定無，虛何曾虛？舉著皆非，故云失措。

This is simply seeing in the present that the aggregate of formations shifts and transforms: within arising there is cessation, therefore it is not existent; within cessation there is arising, therefore it is not non-existent. Through this, he penetrates and understands that everything is neither existent nor non-existent—how could it be limited to eight? "Empty and real" means non-existent and existent: the existent is not determinately existent, so how could the real ever be real? The non-existent is not determinately non-existent, so how could the empty ever be empty? Whatever he points to is negated; hence it is said that "he loses all footing".