And how does a mendicant meditate observing an aspect of the mind?

It’s when a mendicant knows mind with greed as ‘mind with greed,’ and mind without greed as ‘mind without greed.’ They know mind with hate as ‘mind with hate,’ and mind without hate as ‘mind without hate.’ They know mind with delusion as ‘mind with delusion,’ and mind without delusion as ‘mind without delusion.’ They know constricted mind as ‘constricted mind,’ and scattered mind as ‘scattered mind.’ They know expansive mind as ‘expansive mind,’ and unexpansive mind as ‘unexpansive mind.’ They know mind that is not supreme as ‘mind that is not supreme,’ and mind that is supreme as ‘mind that is supreme.’ They know mind immersed in samādhi as ‘mind immersed in samādhi,’ and mind not immersed in samādhi as ‘mind not immersed in samādhi.’ They know freed mind as ‘freed mind,’ and unfreed mind as ‘unfreed mind.’

And so they meditate observing an aspect of the mind internally, externally, and both internally and externally. They meditate observing the mind as liable to originate, as liable to vanish, and as liable to both originate and vanish. Or mindfulness is established that the mind exists, to the extent necessary for knowledge and mindfulness. They meditate independent, not grasping at anything in the world.

That’s how a mendicant meditates by observing an aspect of the mind.