成唯識論述記 4.913

4.913

述曰：謂此二識恒俱轉故，令互相依。量云：其第八識應依於他恒轉之識；以恒起故；如第七識。若不言他即自依不定過，不能自依故。若不言恒轉，云第七識，即無同喻過，第七不依第七識故。若言識性為因，令依第七，即第七前五為不定失，又此恒轉言便無用。今但言依他恒轉識，明令依第七，以七恒轉餘皆間斷故。即此總出依第七也。

[The Commentary states:] This means that because these two consciousnesses always function simultaneously, they are made to rely on each other. The syllogism is: The eighth consciousness should rely on another constantly functioning consciousness; because it arises constantly; like the seventh consciousness. If one did not say "another", it would incur the fallacy of uncertainty regarding self-reliance, since it cannot rely on itself. If one did not say "constantly functioning" and said "the seventh consciousness", it would incur the fallacy of lacking a matching example, because the seventh does not rely on the seventh consciousness. If one used "being of the nature of consciousness" as the reason to make it rely on the seventh, then the seventh and the prior five would be the fallacy of uncertainty; moreover, the words "constantly functioning" would be useless. Now, merely saying "relies on another constantly functioning consciousness" clarifies that it is made to rely on the seventh, because the seventh functions constantly while the others are all intermittent. This generally brings out its reliance on the seventh.