首楞嚴義疏注經 9.55

9.55

此正明也。若帶異計，六行伏惑漸厭漸捨，至福愛天得捨圓融。捨心相續，遍窮捨道。捨心亦亡，不計身心，故名俱滅，此計無想為涅槃也，以此捨心為方便入無想定也。初捨麁心入於微心，復捨微心入微微心，從微微心常修不息，便入無想，此皆修因也。心慮下即無想報，命終果報生無想天，壽命五百大劫，想心不行，故名灰凝。

This directly explains it. If one harbors deviant views, suppressing afflictions through the six practices and gradually reviling and gradually abandoning them, upon reaching the Heaven of the Love of Blessings one attains the harmony of equanimity. The mind of equanimity continues uninterruptedly, thoroughly exhausting the path of equanimity. When the mind of equanimity is also forgotten and one clings neither to body nor mind, this is called 'both extinguished.' This is clutching non-thought as nirvāṇa, using this mind of equanimity as an expedient to enter the absorption of non-thought. First abandoning the coarse mind to enter the subtle mind, then abandoning the subtle mind to enter the extremely subtle mind, and from the extremely subtle mind cultivating constantly without ceasing, one immediately enters non-thought—these are all the causal cultivation. 'Mental deliberation' onward is the non-thought retribution: when life ends, the retribution result is birth in the Heaven Without Thought with a lifespan of five hundred great kalpas; because thinking mind does not function, it is called 'ashes and coagulates.'