首楞嚴義疏注經 3.462

3.462

破兩亦也。同即一也，明暗互滅故云相背，若與一同見即隨滅，如前所破。明、暗、虛空離此無見，亦異不可。

This refutes both being the case. 'Identical' means one. Light and darkness mutually extinguish each other, hence it is said they are 'mutually opposed'; if it were identical with one, seeing would perish accordingly, as refuted before. Light, darkness, and empty space: apart from these there is no seeing, so being different is also impossible.