With respect to principles internally … externally … internally and externally, they meditate observing the liability to originate, to vanish, and to originate and vanish …

If they wish: ‘May I meditate perceiving the repulsive in the unrepulsive,’ that’s what they do. … If they wish: ‘May I meditate staying equanimous, mindful and aware, ignoring both the repulsive and the unrepulsive,’ that’s what they do. That’s how to define the undertaking of the four kinds of mindfulness meditation by a mendicant.”

Linked Discourses 52.2 1. In Private In Private (2nd)

At Sāvatthī. Then as Anuruddha was in private retreat this thought came to his mind: “Whoever has missed out on these four kinds of mindfulness meditation has missed out on the noble path to the complete ending of suffering. Whoever has undertaken these four kinds of mindfulness meditation has undertaken the noble path to the complete ending of suffering.”