Ka+iya (Ka + ॰ईय -īya, the latter forms possesives in Sanskrit) - lit. "with what belongs to Ka".
Prajāpati is the Self (ŚBr. 184.108.40.206).
A Self that wants to become more than one, and desires to reproduce [selves] (ŚBr. 220.127.116.11).
Sakkāyadiṭṭhi is the (wrong) view of a Self/selves that is pervasive and continuous - Moreover, a Self/self (Ka) that is a source of hapiness."Ka" is Prajāpati...
Moreover, the word "ka" is a name of hapiness.
prajāpatir vai kaḥ...
atho sukhasya vā etan nāmadheyaṃ kam iti
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 hapiness.
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)), 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 not breath.
"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:
Kāya is not the physical body (sarīra) - Kāya is the vital function that holds the organs at large.“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 avasissantīti pajānāti.
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 - In first place.
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.
However, the most important thing here, in the first tetrad, is that one gets delight (pīti), and pleasure (sukha) from seclusion.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).
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. https://justpaste.it/17880
Why is it so important to relate Ānāpānasati with Jhāna?“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)
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).
All this to say that, by 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.