大乘起信論義記 2.65

2.65

言非有相者，明真離妄有也。惑者云：既其非有，即應是無？釋云：我非汝妄有，故說非有；非說是無，如何執無？故云非無也。惑者聞上非有，又聞非無，別謂雙非是真如法。釋云：我非汝謂有說非有，非謂法體是非有；非汝謂無說非無，非謂法體是非無。如何復執非有非無？故云非非有非非無也。惑者又云：我上立有立無，汝並雙非。雙非若存，即有無隨喪；今雙非既非，我有無還立。釋云：我非汝雙非故說非非，非許雙是，如何復執？故云非有無俱也。

Regarding the statement that it is 'not the mark of existence', this clarifies that the real is free from delusional existence. A confused person says: 'Since it is not existence, it should therefore be nonexistence.' The explanation states: 'I negate your delusional existence, hence I say it is not existence; I do not say it is nonexistence—how could you grasp at nonexistence?' Therefore it is said: 'It is not nonexistence.' Hearing the above 'not existence' and hearing 'not nonexistence', the confused person separately takes the double negation to be the phenomenon of tathatā. The explanation states: 'I say it is not existence to negate your asserting existence, not to mean that the essence of phenomena is non-existence; I say it is not nonexistence to negate your asserting nonexistence, not to mean that the essence of phenomena is non-nonexistence. How can you once again grasp at neither existence nor nonexistence?' Therefore it is said: 'It is not the mark of neither existence nor nonexistence.' The confused person further says: 'Above, I established existence and established nonexistence, and you doubly negated both. If the double negation stands, then existence and nonexistence perish accordingly; now that the double negation is itself negated, my existence and nonexistence are established again.' The explanation states: 'I say "neither-neither" to negate your double negation, not to admit that both are so—how can you once again grasp [at them]?' Therefore it is said: 'It is not the mark of both existence and nonexistence.'