SuttaCentral
Thus he lives contemplating the body in the body internally, or he lives contemplating the body in the body externally, or he lives contemplating the body in the body internally and externally.
http://www.accesstoinsight.org/tipitaka/mn/mn.010.nysa.html#fnt-4
This is just how I understand it personally, but internally is that part of the body which is taken as a self, as me and/or mine.
External is the stuff that is not takes as a self, as me and/or mine.
Internally and externally leads to seeing that there is no difference between bodies taken to be a self, and bodies not taken to be a self.
For example, there is no difference between the heat from a radiator and the heat felt in the body. There is no difference between the solidity of bark and the solidity of the skin, it is just solid matter.
Contemplating like this removes the distinction between external and internal, causing the body to be seen as not self/me/mine, just stuff.
I feel this view is supported by e.g. MN 140:
“What is the internal earth element? Whatever internally, belonging to oneself, is solid, solidified, and clung-to: head-hairs, body-hairs…
[identification with the body]
Now both the internal earth element and the external earth element are simply earth element .
[But after thinking it through you realized slicing things up in internal and external cant be justified]
And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’
[The body is you as a much rock or a tree is, i.e. not at all]
When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the earth element and makes the mind dispassionate towards the earth element.
[You can’t really do anything useful with solid matter, because it lacks any self-characteristic/self-nature; you don’t control it, it’s not yours, etc.]
Comments in brackets added by me, of course.
Anyway, not presenting a definite answer here, just my own understanding so far, your mileage may vary 
MN 1 will tell you that sometimes the external is also taken to be me, mine or self.
While waiting for the appearance of @chansik_park, I’ll address the easy part on “internally” (ajjhattaṃ).
Ajjhatta = adhi+attan= this self here. Not “Self”, but the non-loaded “person” one takes in a reflexive sense of “myself”.
The “externally” just refers to someone else.
As you rightly point out from that pericope in MN 140, ajjhatta is simply a synonym for paccatta (oneself). Think of “paccattaṃ veditabbo” - to be known by yourself.
I also wish to know the meaning of "body in the body"
Thanks.
It’s an awful translation which creates more misunderstanding than clarity.
The first sentence is best translated as -
Internally, he dwells as a bodyness-contemplator with reference to the body.
“Body in the body” requires “kaaye kaaya.m” which is not what the text says.
I prefer precise Buddhist Hybrid English renderings than imprecise idiomatic English translations, but that’s just me.
The way I understand it is this.
First, the same refrain recurs multiple times throughout the sutta, so we should assume that some of the contemplations described are internal contemplations and some are external contemplations.
The external contemplations must be those in which you contemplate your own body as it would be viewed from the outside. For example, the cemetary contemplations involve thinking of yourself as a rotten and bloated corpse. I suppose one could attempt to call to mind an image of what it would feel like to be a rotten and bloated corpse, if such a thing could still feel. But it makes more sense to interpret this part as an instruction to imagine your rotten and bloated corpse as viewed or seen from some external perspective.
Other objects of contemplations seem to allow or both external and internal perspectives. For example the nails and hair of the head. You can try to “feel” your fingernails or scalp somatically - or at least extend your somatic awareness as far out as it will go to the tops of the fingertips and the scalp. Or you can imagine your nails and hair as seen from the outside.
Then there is whole body awareness. One can internally feel the whole body by suffusing the whole body with somatic awareness, or one can contemplate one’ shifting perspectival body image, or one can do both at the same time. The same is true with just the breath. One can simply attend to he feelings one has in various parts of the body as one breaths, or one can attend to the visual image one tends to form of those parts of the body in which the breathing is felt to be occurring.
For the contrast between internal and external body,
first you should know that MN 10’s citta anupassana text is the exact same standard formula as the mind reading telepathic powers for the 6 higher knowledges (the third one). Just the very beginning is different, explicitly stating the meditator is reading the mind of the other being instead of “internal and external”.
SN 51.12 excerpt
- cetasā cetoparicca pajānāti.
para-sattānaṃ para-puggalānaṃ
other-beings, other-people,
cetasā ceto
their-minds, with-his-mind,
paricca pajānāti.
(he) distinguishes (and) understands.
Sa-rāgaṃ vā cittaṃ ‘sa-rāgaṃ citta’nti pajānāti;
with-passion ** (in their) mind, ‘with-passion (in their) mind’ (he) understands.
… etc.
So from that it’s clear what is meant by internal and what is external from an experiential point of view. Perhaps MN 10 also includes conventional body language observation for those without psychic powers.
Translations into English that employ “body in the body” can appear to permit the impression that there is some kind of Matryoshka-nesting-like meaning behind the phrase as perhaps corroborated in the Anapanasati Sutta MN 118’s ‘body among bodies’.
However, and typing off-the-cuff due to some technical difficulties, I would concur with @Sylvester in that the possibility for conflation brings into what should be understood as iterating a framework bits of a particular implementation, namely Anapanassati.
At the section on locatives in Wijesekera’s Syntax of the Cases, there’s a particular portion that appeared to present a way to address the discrepancy more thoroughly last I looked… (I have, “Wijesekera §174 ff 292” in my notes)
It is. Even Warder translates it as such.
According to Ven. Abhaya it means the contemplation of secondary element of experience.
ie: You experience pain if someone hit you. (first body pain or physical pain). With anger, you can experience this pain over and over again even if you do not have the body pain. (second body pain or mental body pain)
In other words contemplating on the first arrow and the second arrow.
But is it really a choice between these two? When I translate, it is my general assumption that ideas expressed in one language can also be expressed in any other language. This is what I think is behind Warder’s claim that for any full sentence in a given language you should be able to construct a fairly precise equivalent in any target language. (I am paraphrasing here!)
Let’s try this for kāye kāyānupassī. We can perhaps start off with a slightly altered version of your suggested translation: “a body-contemplator in regard to the body.” The exact meaning of this is open to interpretation. It is then natural to ask how this is understood elsewhere, and the obvious place to look is MN 118, which also deals with satipaṭṭhāna. As you are aware, this is what we find there:
Kāyesu kāyaññatarāhaṃ, bhikkhave, evaṃ vadāmi yadidaṃ—assāsapassāsā. Tasmātiha, bhikkhave, kāye kāyānupassī tasmiṃ samaye bhikkhu viharati ātāpī sampajāno satimā vineyya loke abhijjhādomanassaṃ.
Monks, I say this is a certain body among bodies, that is, in and out breathing. Therefore, monks, on that occasion a monk dwells kāye kāyānupassī …
I would interpret kāye kāyānupassī in line with this. In other words, body contemplation involves meditating on a subset of phenomena that are related to the overall body. Since this is my interpretation (and there are others, of course), I would translate as follows: “he contemplates one aspect of the body.”
Using a similar method, I would suggest one can always, or almost always, avoid Buddhist Hybrid English.
I can related this to Chula Vedalla Suta as well. In and out breath is bodily fabrication. That means you contemplate on your body (blood, skin, flesh etc.) as well as its fabrications (breath and other movements etc.)
Indeed Bhante! As usual, I’m mindful of the Comy as well as Ajahn Brahm’s “bird-watching birds” explanation.
My grouse was with the “body in body” translation. Although B&B is quite clear in his reason for this rendering (see fn 197 in his AN translation), it is apt to mislead some readers to read this literally to mean that there’s a Russian dolls phenomenon going on with bodies nested within bodies.
The problem starts with translators who see a periphrastic construction in the phrase, which they render as if “dwells” is an auxiliary verb to a presumed gerund/absolutive "contemplating ". But anupassin is not a verb but an adjective, so this does not qualify as periphrasis. What we have is anupasii = the person that is anupassin = the contemplator.
This is where the next mistake arises. Kaaya is not standing in a relationship with the kaaya in kaayaanupasii, but the anupasii.
Now all this is old news to Bhante and BB, but for someone who does not realise that BB did not really mean a spatial/local locative for kaaye, they would need to be familiar with the MN 118 correspondence to understand the meaning of kaaye kaayaanupasii viharati. Then, disputes would start if one rejects MN 118, and insists that the mindfulness refrain should be read literally as translated.
Put this way - would BB’s idiomatic translation be more likely to give rise to misunderstanding than the literal translation he offers in said fn .197?
Would Bhante comment on translating the refrain as “he contemplates aspects of the body”? I’ve opted for a plural.
Bhante how can you relate this to other three. (feeling ,thoughts and Dhamma)
Can you give an example?
Yes, I see what you mean with Ven. Bodhi’s translation. Even if he had just changed the “the” to an “a,” things would have been a bit clearer: “… dwells contemplating a body in the body.” But only a bit!
I think your idea of translating with the plural is interesting. All the phenomena contemplated belong to sets. On top of this the plural is as justifiable as the singular on grammatical grounds.
Keep up your good work!
I would translate it in a parallel fashion:
he contemplates one aspect of feeling …
he contemplates one aspect of mind …
he contemplates one aspect of conditionality …
The last one is experimental.
It appears that we should understand the meaning of Anupassana.
According to Ven Abhaya, Anupassana means cessation by contemplation on Anicca, Dukkha and Anatta. According to him, we can’t cease the five aggregate but we can cease only the clinging-aggregate.
In another video Ven. Abhaya explained that contemplating on the five aggregate and the clinging-aggregate is meant by the body in the body etc.
In the following video by Ajhan Lee, body in the body means contemplate on one element of the aggregate. Kaya means the aggregate of earth, wind, air and heat elements.
Contemplate on air (wind element) only termed contemplate body in the body.
Then you contemplate on the refine aspect of the wind element. (hard, soft etc)