成唯識論述記 7.407

7.407

述曰：此論主答。五識俱意助五識令起，由意引五方得生故，非專為了五所緣，故與五同緣者。不同緣者，便不能引眼等識中三性等生故。

The Commentary states: This is the author's answer. The manas co-arising with the five vijñānas assists the five vijñānas to cause them to arise, because the five are only able to arise due to the manas inducing them; it is not solely for the purpose of discerning the cognitive objects of the five that it shares the same cognitive object with the five. If it did not share the same cognitive object, it would not be able to induce the arising of the three natures and so forth within the eye-vijñāna and so forth.