大毘盧遮那成佛經疏 2.38

2.38

第九「云何闇心？謂於無疑慮法，生疑慮解」者，謂如四諦不淨無常等，世間智者不應生疑，然彼聞之心懷猶豫，如夜見株机生種種憶度之心。若見有如是相者，當知暗心使然也。

Ninth: regarding "What is the dark mind? It means giving rise to an understanding of doubt and suspicion regarding dharmas that are free of doubt and suspicion": this means that regarding things such as the Four Truths, impurity, and impermanence, wise people in the world should not give rise to doubt; yet when this person hears them, they harbour hesitation in their mind, like seeing a tree stump at night and giving rise to various conjectures. If one sees such a mark, one should know that it is brought about by the dark mind.