In the context of this sutta, when one is moving through the Buddha’s instruction and physical tension, mental sluggishness or mental scatteredness of the mind arises, how would the Buddha exactly define “an inspiring foundation”?
“That’s so true, Ānanda! That’s so true! Any monk or nun who meditates with their mind firmly established in the four kinds of mindfulness meditation can expect to realize a higher distinction than they had before.
What four? It’s when a mendicant meditates by observing an aspect of the body—keen, aware, and mindful, rid of desire and aversion for the world. As they meditate observing an aspect of the body, based on the body there arises physical tension, or mental sluggishness, or the mind is externally scattered. That mendicant should direct their mind towards an inspiring foundation. As they do so, joy springs up. Being joyful, rapture springs up. When the mind is full of rapture, the body becomes tranquil. When the body is tranquil, one feels bliss. And when blissful, the mind becomes immersed in samādhi. Then they reflect: ‘I have accomplished the goal for which I directed my mind. Let me now pull back.’ They pull back, and neither place the mind nor keep it connected. They understand: ‘I’m neither placing the mind nor keeping it connected. Mindful within myself, I’m happy.’