成唯識論述記 8.193

8.193

述曰：其第六識於前五識無此緣義，五識於六有此緣義，五識緣本識所變為境，不待第六識所變色等為自境故，有力生識者方為此緣故，不以五識自相望為所緣緣者，以定無故。

The Commentary states: The sixth consciousness lacks the sense of being this condition in relation to the previous five consciousnesses, whereas the five consciousnesses possess this sense in relation to the sixth. The five consciousnesses take that which is transformed by the root consciousness as their cognitive object and do not depend on the form and so forth transformed by the sixth consciousness as their own cognitive object; for only that which has the efficacy to generate a consciousness can serve as this condition. The reason the five consciousnesses are not considered objective conditions mutually among themselves is that they definitely lack it.