首楞嚴義疏注經 9.101

9.101

「識性不動，以滅窮研，於無盡中發宣盡性，如存不存、若盡非盡，如是一類名為非想非非想處。」

"The nature of consciousness being unmoving, one exhaustively investigates through extinction; within the unending, one makes manifest the nature of exhaustion, as if existing yet not existing, as if exhausted yet not exhausted. Such a class is named the Sphere of Neither-Perception-Nor-Non-Perception."