May I again relate kāya to the god "Ka", in middle-late Vedic philosophy.
Ka+iya (Ka + ॰ईय -īya, the latter forms possessives in Sanskrit) - lit. "what belongs to Ka".
Prajāpati is the Self (see ŚBr. 126.96.36.199).
A Self that wants to become more than one, and desires to reproduce [selves] (see ŚBr. 188.8.131.52).
"Ka" is Prajāpati...
Moreover, the word "ka" is a name of happiness.
prajāpatir vai kaḥ...
atho sukhasya vā etan nāmadheyaṃ kam iti
Sakkāyadiṭṭhi (saṃ+ka+iya+diṭṭhi - lit. "with what belongs to Ka"), is the (wrong) view of a Self/selves that is pervasive and continuous - Moreover, a Self/self (Ka) that is a source of happiness.
The Buddha says that this Self/self cannot exist in paṭiccasamuppāda, because there is impermanence - and impermanence is not continuity; and that non-continuity is dukkha.
The opposite of the late Brāhmṇa/Āraṇyaka view of a continuous (pervasive) Self/self that brings happiness.
If the god "Ka" was still to exist in paṭiccasamuppāda - that is to say, if the word "kāya" had not passed in the current language as the somewhat vital function of the body (sarira) - the only happiness of "Ka", would be the happiness derived from the breath (prāṇa) - viz. pīti and sukha - not from "Ka" as the Self/self.
What does the breath has to do with all that?
Kāya, (what belongs to Ka,) is closely related to the organs of the self.
"Organs", or more precisely here, their "vital functions" (speech, sight, hearing, mind (mano)), which, in Indian Vedic philosophy, encompasses also prāṇa (breath) (ChUp. 5.1.6).
Breath is the highest organ. The one that never stops during the life time. One can lose his sight, or hearing, and not die - but one cannot lose breath, and not die.
"The one,after whose departure the body (sarira), appears to be in the worst shape, is the greatest among you", says Prājapati, to the vital functions competing between them.
So breath wins.
The attributes of Ka are these organs, with their vital functions. And the highest is breath. And speech is pretty closely related to it.
In Buddhism, the word Kāya deals with what does not pertain to eye, ear, nose, mouth and mind (mano). And it is definitely breath (prāṇa*).
*Prāṇa, in the singular refers to breath - and in the plural, to the vital functions/powers at large.
Yet, Kāya is also what "glues" all the organs.
Kāya might be better understood in the following:
“When he feels a feeling extending as far as the body (kāya), he understands: ‘I feel a
feeling extending as far as the body.’ When he feels a feeling extending as far as the principle of life
(jīva), he understands: ‘I feel a feeling extending as far as the principle of life.’
He understands: ‘With the breakup of the body, following the exhaustion of life, all that is felt, not
being delighted in, will become cool right here; mere bodily (sarīra) remains will be left.
“so kāyapariyantikaṃ vedanaṃ vedayamāno kāyapariyantikaṃ vedanaṃ vedayāmīti pajānāti,
jīvitapariyantikaṃ vedanaṃ vedayamāno jīvitapariyantikaṃ vedanaṃ vedayāmīti pajānāti.
kāyassa bhedā uddhaṃ jīvitapariyādānā idheva sabbavedayitāni anabhinanditāni sītībhavissanti, sarīrāni
Kāya is not the physical body (sarīra) - Kāya is the vital function that holds the organs at large.
Kāya is (somewhat) to sarira, what sight is to the eye. I say somewhat; because kāya has a wider range than a simple internal ayātana. It has a magnitude that goes beyond the Buddhist "world" (of senses).
In Buddhism, breath is a crucial factor. It comes right in Saṅkhāra nidāna, the second link of
Paṭiccasamuppāda - Ranking first.
It is a bodily formation (kāyasaṅkhāra). And it leads to feeling.
This is what ānāpānasati is supposed to reenact; at satta's (man) level.
Let's get to the first tetrad in ānāpānasati.
The fact that the Saṃyukta-āgama (SA 810) insists on the fact that the breath is the exclusive object of mindfulness in the first tetrad, is not as evident in SN 51.3.
What is the scope of that object of mindfulness, is breath and the "body" (kāya).
They are intricately interrelated through the saṅkhāra (kāyasaṅkhāra).
The noteworthy difference in relation to the third step of ānāpānasati, is that whereas the MN 18 (nikaya) speaks of experiencing the “whole body”, the SA 815 (agama) counterpart speaks of experiencing “all bodily formations”.
But there is no conflict into that.
Sabbakāyapaṭisaṃvedī assasissāmīti sikkhati.
Pali: Trains oneself
Sanskrit: śikṣati - inflected form - शक् śak) is a desiderative verb that has the underlying meaning of
"desiring to be able to". It is about training, with the "desire to be able to".
√ विद् vid : to know | to understand | to have the feel of, to be conscious of | to see.
प्रतिसंविद् pratisaṃvid : An accurate knowledge of the particulars of anything.
प्रतिसंवेदिन् pratisaṃvedin [prati+saṃ+vedin] : being conscious of anything, feeling, experiencing.
vedin: e.g. āyurvedin = expert in ayurveda (medecine).
However, the most important thing here, in the first tetrad, is that one gets delight (pīti), and pleasure (sukha) from seclusion.
This is not in ānāpānasati per se; but in the Jhāna process. But it will explain better what "established mindfulness in front of him" means.
What does seclusion (viveka) mean?
Viveka means: Discrimination between the external & the internal - separation from the external - and then seclusion in the internal. (see here).
“Desiring seclusion you entered the woods,
Yet your mind gushes outwardly.
“You must abandon discontent, be mindful.
“vivekakāmosi vanaṃ paviṭṭho,
atha te mano niccharatī bahiddhā.
aratiṃ pajahāsi sato.
SN 9.1 (SA 1333)
Why is it so important to relate Ānāpānasati with Jhāna?
First, because it is the seclusion (viveka) that brings the pīti & sukha paṭisaṃvedī (5th & 6th steps).
And secondly, because, it is this the pīti & sukha paṭisaṃvedī that brings the ultimate concentration of the 11th step (samādha citta).
In other words, seclusion, (which encompasses mindfulness, abandonment of discontent, etc with the external), is a prerequisite to experiencing feeling. And contentment (pīti & sukha,) "born of seclusion" is the prerequisite to know the citta. Samādhi (https://justpaste.it/zcue) of breath, being the goal and the unifying thread towards that goal of "perceiving the feeling" (aka citta).
Sticking strictly to the commonalities in the Nikayas & the Agamas (in the EBTs), one can understand exactly what is the process to follow. And, by that, reach right concentration.
The all process is in what Buddha strictly said. No more, no less.
KĀYA as BREATH
Let's see what is given in SN 41.6:
[size=85][b]1. In-breathing and out-breathing are bodily, these things are bound to the body; that is why in-breathing and out-breathing are the bodily formation [synergy].
Assāsapassāsā kho āvuso visākha kāyikā ete dhammā kāyapaṭibaddhā. Tasmā assāsapassāsā kāyasaṅkhāro.[/b][/size]
Are in&out breathes made "by" the body (kāya)?
Are in&out breathes made "for" the body (kāya)?
What one has to do first, is to go to the definition of the second saṅkhāra, viz. vacīsaṅkhāra, to understand the meaning of the definition of kāyasaṅkhāra.
First one thinks abstractly (vitakka), and then thinks concretely (vicāra); then afterwards one breaks into a word; that is why abstract thinking & concrete thinking are the verbal formation.
Pubbe kho āvuso visākha vitakketvā vicāretvā pacchā vācaṃ bhindati. Tasmā vitakkavicārā vacīsaṅkhāro.
We know that "vacīkamma" means "deed by words".
But we can't say that abstract thinking (vitakka) and concrete thinking (vicāra) are formation "by" speech (word).
Because the thinking comes before the word.
Vacīsaṅkhāro has to be translated as: "formation for the word" ("for" speech).
The same way that in&out breathes are made "for" the body (kāya)? - &
The same way that vedanā &sañña are made "for" the mind (citta)?
Note that, across Buddha's time, in the Gṛhya-Sūtra (GṛŚrS.,) and the Mahābhārata (MBh.), saṃskāra (संस्कार) means "putting together".
This kind of meaning - like "making up (from parts)" - is also found in the ŚBr. or the BṛĀr.Up., etc.
In other words, one puts the in&out breathes, "for" the Kāya. As a synergy (saṅkhāra).
KĀYA as ĀYATANA
As an āyatana, there is also a non-material side of Kāya.
The field of experience that is an āyatana, involves more than the physical part of the "organ" that is the physical "body".
There is just more than the sensory experience of "making a breath".
Here the body is also a sensory moment, of a bodily sensory experience, on the field of experience that is the body āyatana.
When a cool and nice wind passes on one's skin on a hot afternoon - and beforeone experiences a pleasant feeling - the field of experience, that is one's body āyatana, has a sensory moment - an arising of that field, proportional to the indriya one has at the time (restrained or not).
Again, I like to equate that to the metaphor of the āyatana as the muscle of the frog, that is triggered by the current (indriya) to produce a reflex (feeling). One does not apply the current to the reflex. It has to go through the muscle.
In the same way, the field of experience (āyatana) has a sensorial moment, proportional to the intensity of the indriya. And it is that sensorial moment that trigers the sensory experience (sense consciousness >> contact >> feeling, etc.
Note that one does not build a whole physical body every time he does experience a bodily sensory experience. It is just the sensorial moment of that bodily sensory experience (on the immaterial body āyatana) that arises and fades (impermanence).
This triggers a feeling (etc.) that is also impermanent.