大般涅槃經義記 9.194

9.194

就明疑中，迦葉初言「如是人者本自不疑，云何說言不壞疑網」乘前為問。佛前說言執著之人不壞疑網，故名不善，故今問之，此人由來於法不疑，云何不壞？下佛答之。夫不疑者於正義中心不決了，即是疑也。

Within clarifying doubt, Kāśyapa first says, 'Such a person originally had no doubt; why do you say that they do not break through the net of doubt?', taking up what came before to formulate a question. Because the Buddha earlier said that a person who clings does not break through the net of doubt and is therefore called unwholesome, [Kāśyapa] now asks: this person has never had doubt regarding the Dharma, so why are they said not to break through it? Below, the Buddha answers. One who does not doubt lacks decisive understanding in their mind regarding the correct meaning—this itself is doubt.