首楞嚴義疏注經 3.177

3.177

破共生也。若根境合生中界者，此識中界知、不知別，故云中離。若成別者，此識中界一半合根、一半合境，故云兩合。兩合若成，有雜亂過，知與不知同一界故，界義應非。或離者開義，或猶無也。

This refutes joint arising. If the sense faculty and the objective realm unite to produce the intermediate realm, this intermediate realm of consciousness would differ between knowing and not knowing; hence it says 'the middle is separated'. If they form distinct parts, then half of this intermediate realm of consciousness would unite with the faculty and half would unite with the realm; hence it says 'they doubly unite'. If a double union were formed, there would be the fault of mixture and confusion, because knowing and not knowing would share a single realm, so the meaning of 'realm' would be invalidated. Alternatively, 'separated' (li) has the sense of opening apart, or resembles 'nonexistent'.