Taking the beginner’s necessarily linear interpretation of the Anapanasati sutta (MN 118), the first tetrad of the physical body leads to the second, feelings, which arise as bodily sensations. Thanissaro's approach to breath meditation focusses on the arising of pleasant feelings, and is opposed to the visualisation of the sign presented in the Visuddhimagga. The tactile skill of recognizing where the breath touches parts of the body is easier and more elementary than the skill of visualization:
“The commentaries—molding the practice of breath meditation into the
pattern of kasina practice, in which the mind has to become focused exclusively
on a single point—insist that “body” here means the breath, and that the “entire
body” means the entire length of the breath, felt at one spot in the body, such as
the tip of the nose or the upper lip.
This interpretation, however, is unlikely for several reasons. The first is that
the commentaries’ interpretation of step 3 makes it redundant with steps 1 and 2.
It’s hard to understand how you could know whether the breath is long or short
in those steps without being aware of the full length of the breath.
The second reason is that step 3 is immediately followed by step 4, which—
without further explanation—refers to the breath as “bodily fabrication.” If the
Buddha were using two different terms to refer to the breath in such close
proximity—“body” in step 3, and “bodily fabrication” in step 4—he would have
been careful to signal that he was redefining his terms (as he does in a later part
of the discourse, when explaining that the first four steps in breath meditation
correspond to the practice of focusing on the body in and of itself as a frame of
reference). But here he doesn’t.
The third reason is that the similes for the jhanas, which are attained through
the sixteen steps, repeatedly mention a full-body awareness. If the mind were
forced exclusively into a single point, it wouldn’t be able to spread feelings of
rapture or pleasure throughout the entire body in the first three jhanas, or to fill
the body with a clear bright awareness in the fourth.
One response to this last argument is that the word “body” in the similes for
jhana doesn’t mean the physical body, because a person in jhana has to be
oblivious to the physical body. Instead, “body” is meant metaphorically as a
term for the “body” of the mind.
Putting aside the question of why someone with the Buddha’s teaching skills
would use terms in such a potentially confusing way in his basic meditation
instructions, we can simply note that in MN 119 he gives the similes for the
jhanas immediately after his discussion of six ways of focusing on the physical
body. As in the case of steps 3 and 4 in breath meditation, if he had meant
“body” to mean “physical body” in one context, and “mind body” in the
discussion immediately following it, he would have signaled that he was
redefining his terms. But again he doesn’t.
So unless we want to assume that the Buddha was careless or devious in his
meditation instructions, it seems best to interpret “body” as meaning “physical
body” in all of these contexts, and to interpret “entire body” in step 3 as
referring to the entire physical body as sensed from within.”—-“Right mindfulness”, Thanissaro.