About anapanasati, there are many modern teachers talking about this but I have only read Buddhadasa comment extensively on the key phrase found in the sutta, namely: "He trains himself".
The same as Buddhadasa, the suttas define the term "training" to refer to the Threefold Training, as follows:
There are these three trainings. Which three? The training in heightened virtue, the training in heightened mind, the training in
AN 3.88 Sikkha Sutta: Training
This below from his 550 pages is merely one point providing evidence to a hypothesis that Buddhadasa taught anapanasati correctly and every one else taught anapanasati incorrectly.
Related to the point above, the Buddha said he teaches about suffering & the cessation of suffering (MN 22). Suffering is created by the mind rather than by the body. Since the phrase: "He trains himself" must include insight into suffering & the ending of suffering, that is, the four noble truths, I have also only read Buddhadasa teach this from step 3, as follows:
This above from his 550 pages is merely a 2nd point providing evidence to a hypothesis that Buddhadasa taught anapanasati correctly and every one else taught anapanasati incorrectly.
Below from his 550 pages is a 3rd point providing evidence to a hypothesis that Buddhadasa taught anapanasati correctly and every one else taught anapanasati incorrectly. I have never read the following explanation by any other teacher; yet the explanation below accords with the principles in the suttas about feeling & perception.
Buddhadasa explains this more clearly in his later work, below:
EXPERIENCING THE MIND-CONDITIONER
Now we come to step seven: "experiencing the mind-conditioner (cittasankhara-patisamvedi)." If we have completed step six
successfully, then we know all about the feelings of piti and sukha.
What does the arising of piti do to the citta? What does the arising
of sukha do to the citta? What kind of thoughts does piti condition?
What kind of thoughts does sukha condition? We have noted and
scrutinized these effects since steps five and six. Once we come to
step seven, it is easy to realize that, "Oh, piti and sukha are
mind-conditioners." These vedana are mind-conditioners in the same way
that the breath is the body-conditioner. The method of study and
observation is the same as in step three. (107)
We have observed that piti is coarse and excited, whereas sukha is fine and peaceful. Thus, when piti conditions or brews up a thought,
the thought is coarse. On the other hand, when sukha brews up a
thought, it is calm and tranquil. This is how we realize that the
vedana condition thoughts. Then we realize that the feelings condition
both coarse thoughts and subtle thoughts. We call this activity
"conditioning the mind." (108)
When piti is strong, it causes trembling in the body. And if it is very strong the body might even dance or bounce with joy. This
feeling is coarse and powerful. On the other hand, sukha is calming,
soothing, and relaxing. We learn that their characteristics are very
different. When piti dominates the mind, it is impossible to think
subtle thoughts. We feel a tingling all over; it makes the hair stand
up all over our bodies. So we need to be able to control piti. Sukha,
however, has advantages. It leads to tranquil, refined states. It can
cause subtle, profound, and refined thoughts. It is as if these two
feelings are opponents or foes. But that does not matter, for we know
how to regulate them. We are able to control them by training
according to the method we are now practicing. Just this much is to
understand the citta-sankhara reasonably well already.
In the end, we will realize that the feelings (e.g., piti and sukha) are mind-conditioners. When piti conditions it, the citta is coarse
and its thoughts are coarse, both the mind and the thoughts are
coarse. When sukha conditions or supports it, the citta is subtle and
tranquil, and its thoughts are subtle and tranquil. Both feelings
condition the mind, but from different angles. The vedana are
conditioners of the citta, thus they get the name "mind-conditioner
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