大毘盧遮那成佛經疏 2.54

2.54

Twenty-fifth: regarding "What is the well mind? It means contemplating thus: 'Deep and yet deeper'": just as looking down into well water, the measure of its depth is difficult to know. The nature of this mind is also like this: whatever it contemplates, it likes to make deep and distant; in all matters, wholesome or unwholesome, it always desires to make people unable to fathom them, so that even those who practice and dwell with them and engage in the same undertakings do not know their mental activities; one should know that this is the well mind. The Dharma-gateway of dependent origination and the marks of good people are all clear and easily understood; this is its antidote.