This is a short study note, not a talk. It draws together a few canonical passages on one phrase from the Satipaṭṭhāna Sutta, and is offered as a teaching aid rather than a scholarly claim. Where I give my own reading, I say so. The sources are at the end; I'd encourage you to read them directly.
What the Words Mean
Ajjhattaṃ means internally, what is within. Bahiddhā means externally, what is without. Together with ajjhattabahiddhā, both internally and externally, they form a three-part instruction that returns in every section of the Satipaṭṭhāna Sutta (MN 10).
One dwells observing the body as body, internally, or externally, or both internally and externally.
The same refrain is repeated for feeling tone, mind and dhammas. So this is not a minor detail. It sets the full scope of the practice across all four foundations.
The sutta itself doesn't define the pair. Teachers and traditions have read it in more than one way. The reading below is the one I find most useful in practice, because it is grounded directly in another part of the Canon.
Faculty and Object
In the Saḷāyatana Saṃyutta (SN 35), the Buddha speaks of the six internal sense bases (eye, ear, nose, tongue, body and mind) and the six external sense bases (sights, sounds, smells, tastes, touches and mental objects). The words there are ajjhattika and bāhira, close relatives of the pair in MN 10 rather than the same words.
Where a faculty meets its object, there is contact. From contact, feeling tone arises, and from feeling tone, craving can follow. This meeting point is where experience actually forms, and so it is where the practice does its work.
In the talk I described it as the difference between what reaches you and what you do with it. Something arrives at a sense door. Something happens inside because it arrived. Seeing those as two, and not one, is much of the practice.
Why Internal Comes First
The Buddha's teaching keeps returning to what lies within the practitioner's direct reach. The Dhammapada puts it simply: attā hi attano nātho, one is one's own protector (Dhp 160). This is not self-absorption. You can only observe directly what you can actually access.
The Sedaka Sutta (SN 47.19), the story of the two acrobats, shows where this leads. Protecting oneself through the practice of awareness, one protects others. Protecting others through patience, harmlessness, goodwill and kindly concern, one protects oneself.
Not Clinging
Each section of the sutta ends with the same lines:
One dwells independent, not clinging to anything in the world.
That is where the instruction points. Whatever is seen, inside or out, is known as it arises and passes, and let be.
Sources behind this note
Numbers follow the standard Pali Canon divisions, so you can look them up in any translation.