成唯識論述記 7.401

7.401

述曰：第四問答。外人難曰：何故諸識中以眼識等自同類識不俱起也？此難即令一念之時有二眼等識義，由許八識俱起故。

The Commentary states: The fourth question and answer. The opponent challenges: Why is it that, among the vijñānas, vijñānas of their own category—such as the eye-vijñāna and so forth—do not co-arise? This objection implies that there would be two eye-vijñānas and so forth in a single thought-moment, since eight vijñānas are allowed to co-arise.