成唯識論演祕 6.152

6.152

五俱意識而有二義。一明了取。二能助五。若二眼識雖有明了。無助引生故不為例。如定意識雖實復明了不能助五。

The mental consciousness concurrent with the five [sense consciousnesses] has two meanings: first, clear apprehension; second, being able to assist the five. Although two eye-consciousnesses would have clear apprehension, they lack the ability to assist and induce arising, and therefore do not serve as an analogy. Just as mental consciousness in meditative concentration, though actually clear, cannot assist the five.