I hope this isn’t too jumbled as a reply. I tried to write something more like an essay and gave up when the “proliferation” went too far. Mostly, I think kāyagatāsati didn’t originally mean “mindful of one’s body” but “mindful of oneself.” But clearly that isn’t the case in many Theravāda readings.
It looks to me like the Theravāda tradition has multiplied the number of passages in which kāyagatāsati occurs, which is something that appears to have happened with the similes about the “body” being drenched in joy and happiness as well. This probably happened because the two things - the abodes of mindfulness and kāyagatāsati - were conflated as synonymous. Or something. It’s hard to tell.
None of the Chinese parallels support the occurrence of kāyagatāsati in this passage. EA 38.8 doesn’t even mention “body.” It’s simply focused mindfulness and an undistracted mind (念專精,意不錯亂) that controls the six senses at the conclusion of the sutra. SA 1171 mentions the first abode of mindfulness.
In SA 1175, there are four gates, one for each of the four abodes of mindfulness.
I think that kāya sometimes functioned as a shorthand for oneself or the whole person. That’s certainly the case in Chinese, but some passages in Indic show that pattern, too. So, I think “mindfulness of body” sometimes could mean something as natural as “mind yourself” in practice. Theravādins have fallen into literalism with kāya, but reading it that way renders some passages incomprehensible. What is “body witnessing”? In Chinese, it clearly means a personal realization, as opposed to taking someone else’s word for it, which is an important theme for Buddhists. The third jhāna is the same. It’s personally experiencing for oneself what noble sages talk about experiencing.
We have to remember that there was an ideological problem with the concept of a “person” or “self.” The concept had to hide behind other words like kāya or nāmarūpa after pudgala became anathema. And that word was itself a replacement for a modified concept of atma. So, I think Theravādins, like the other anti-Pudgalavādins, suffered from philological distortions as a result, and this is still happening to this day. People argue incessantly about kāya and avoid the obvious reading that it sometimes refers to oneself as a whole. The result is that the original meaning of some passages is obscured by ideological angst, as it were. At least, that’s my working theory.
Again, EA 37.6 doesn’t mention mindfulness of “body,” but just a mindful or focused mind. In MA 24 and AN 9.11 I have to wonder how we would understand the expression kāye kāyagatāsati if we read kāya as just the physical body. Doesn’t it simply mean “one who is mindful of themselves”? Someone who isn’t mindful of themselves can easily break precepts. It makes sense to me, and that’s the way EA 37.6 reads, which isn’t so formalized a text as the AN and MA versions. But someone has decided to reinterpret the sutra to refer specifically to mindfulness of body exercises, and then it becomes confusing. This may have been because one of the last analogies in which Sāriputra says he is repelled by his body. That occurs in EĀ 37.6, but it’s the only place in the entire sutra that “body” is mentioned. The rest of the analogies don’t suggest he is talking about body mindfulness to me. It makes much better sense to read it as being mindful or oneself and accepting of whatever is encountered, which eliminates hostility.
There’s a similar expression used in other sources when describing mindfulness of breathing, such as EĀ 17.1. I think it must refer to the happiness one feels when they’ve established themselves in a state of samādhi and have been released from the hindrances and afflictions (at least temporarily).
The expression 正念樂住 only occurs in this one sutra. If I search for 念樂住, what I find is the third jhāna … the meaning is probably the same as I suspect for the mindfulness of breathing. Being in mindful samādhi is a happy time because the causes of suffering have been suspended.
There’s also an interesting passage in SA 615 (SN 47.10) that puts more of a gloss on the expression:
心定者,聖弟子當作是學:『我於此義,外散之心攝令休息,不起覺想及已觀想,無覺無觀,捨念樂住;樂住已,如實知。』
Their mind being concentrated, the noble disciple will train in this way: “In regard to this subject, the outwardly distracted mind has been collected and made to stop. Not producing feelings and ideas and having examined ideas, there is no perception or investigation. Detached and mindful, it abides happily. Having abided happily, it’s truly known.”
So, yes, I think it refers to the happiness of having put the mind to rest all the outside distractions and afflictions, which also results to clear thinking.
The Chinese equiv. 身作證 occurs in many contexts and clearly means “to realize for oneself” or “personally realize.” “Body witness” is simply a bizarre reading to me. It’s another example of how this literalism renders passages incomprehensible.
念其身 and 念隨身 could be translations of kāyagatāsati. There’s a qualifier like upanibandha that’s been added. Which is par for the course when comparing Sarvāstivāda and Theravada texts.
But reading it as “body” doesn’t make sense for some of those exercises. It may have originally meant “one’s actions” in general.