大毘盧遮那成佛經疏 2.39

2.39

第十「云何明心？謂於不疑慮法，無疑慮修行」者，謂於決定法印非可疑慮之法，彼隨所聽聞即能懸信，當知是明心也。然是中若過若不及，即是障道之心，更處中慧是彼對治。

Tenth: regarding "What is the bright mind? It means cultivating without doubt or suspicion regarding dharmas free of doubt and suspicion": this means that regarding dharmas that are determinate dharma-seals and not to be doubted or suspected, as soon as this person hears them they are able immediately to believe them; one should know that this is the bright mind. However, if there is excess or deficiency in this, it is a mind that obstructs the Path; cultivating the wisdom of the middle way is its antidote.