大般涅槃經集解 65.84

65.84

案僧亮曰：謂執者於所執不疑也。不疑即是疑者，佛語皆實不虛，雖復偏執不疑，已為二說所壞，信不成就，即是疑也。

Sengliang comments: This refers to the grasper having no doubt regarding what is grasped. As for having no doubt being itself doubt: the Buddha's words are all true and not false; although one one-sidedly grasps without doubt, it is already ruined by the two teachings, and faith is not achieved—this is precisely doubt.