成唯識論述記 6.576

6.576

述曰：此釋愛、疑不得俱起。愛境必決，疑必不定，故貪、疑不俱。上說不俱，下說俱者。

The Commentary states: This explains why attachment and doubt cannot arise together. The object of attachment must be determined, whereas doubt is necessarily undetermined; hence attachment and doubt do not occur together. Above was explained their non-co-occurrence; below explains their co-occurrence.