A question on MN 39 Mahā- Assapura Sutta

Having never read MN 39 before, my first impression is we must try to examine precisely for unique aspects.

Therefore, in relation to purity of body, speech & mind, this appears a practise performed communally in relation to other monks. The text says:

Our bodily, verbal & mental conduct will be pure, clear & open, unbroken & restrained. We will not exalt ourselves nor disparage others on account of that pure bodily, verbal & mental conduct... (Thanissaro translation)

You should train like this: ‘Our bodily, verbal & mental behavior will be pure, clear, open, neither inconsistent nor secretive. And we won’t glorify ourselves or put others down on account of our pure bodily, verbal & mental conduct.’ (Sujato translation)

The above appears related to how monks relate to eachother, including making confession to eachother, including for their unwholesome mental states. For example, giving another monk merely a disparaging look would be an example of impure mental conduct. The suttas clearly say monks should look at each other with "loving eyes"; blending like milk & water (MN 31).

“I hope you’re living in harmony, appreciating each other, without quarreling, blending like milk and water, and regarding each other with kindly eyes?”

MN 31

Therefore, the above three aspects of 'purity of conduct' appear explicitly separate from the practises described afterwards, such as abandoning the five hindrances. Abandoning the five hindrances may be explicity an "internal" matter/practice; while what is called "purity of conduct" may be an "external" matter related to communal/social conduct.

The above is my impression of the text.

Therefore, 'restraint of the senses' includes when a monk is on alms round and sees a scantly dressed or topless lady, as was/is common in traditional cultures. This example is given (somewhere) in the suttas. This context of practice of restraining the senses on alms round is different to the purity of mental conduct a monk develops towards other monks.

To continue, a monk may be skilled at purity of conduct towards other monks but have sloth & torpor when it comes to personal meditation practice. Thus this monk must develop wakefulness.

The development of jhana is even more distinct. The Buddha himself taught jhana is developed by making 'letting go/surrender' ('vossagga') the meditation object (SN 48.10; end of MN 118). Therefore, a monk may be able to abandon the five hindrances however they may develop a mind of 'rigid imperturbability' ('āneñja') rather than a mind of 'letting go/surrender to the void' ('vossagga').

Note: later texts plus suttas such as SN 12.51 refer to 'imperturbability' ('āneñja') as an non-Noble non-Path mental state.

Also, the development of jhana may not necessarily automatically result in the fruition of the Three Knowledges. The text says:

With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, he directs and inclines it to knowledge of the recollection of past nivasa (dwellings; abodes; not "lives").

The practice of recollection of past abodes is only fully explained in SN 22.79.

Therefore, the impression of the above is a deliberate act of mind is required to direct the mind towards recollection of past abodes rather than the opposite, which would be to remain in jhana and develop the immaterial spheres.

Note: MN 140 says about the 4th jhana & this step:

One discerns that 'If I were to direct equanimity as pure & bright [of the 4th jhana] as this towards the dimension of the infinitude of space and to develop the mind along those lines, that would be fabricated. One discerns that 'If I were to direct equanimity as pure and bright as this towards the dimension of the infinitude of consciousness... the dimension of nothingness... the dimension of neither perception nor non-perception and to develop the mind along those lines, that would be fabricated.' One neither fabricates nor mentally fashions for the sake of becoming or un-becoming. This being the case, one is not sustained by anything in the world (does not cling to anything in the world). Unsustained, one is not agitated. Unagitated, one is totally unbound right within. One discerns that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for this world.'

In conclusion, my impressions above show MN 39 may not be a straightforward as it may appear.

Whenever one analyses ancient texts, it's worth remembering two points:

1. The texts were passed down (for that main part) orally, and may not have been formally organized and written for several generations. This introduces recitation artifacts.
2. The recipients of these texts were largely uneducated, if not actually illiterate, so teachings leaned on simple, clear messaging, not the niceties of philosophical abstraction.

The texts we ultimately receive are an admixture of the Buddha's teachings and subsequent techniques of rote learning that were used to pass the teachings down faithfully. For those of us in the modern world — at least those of us in the industrialized world, which have high literacy rates and comparatively sophisticated education systems — they might seem oddly oratorical and repetitive. But those features are precisely what made transmission of the text so successful in the ancient world.

With respect to MN 39... Yes, your instinct is right that cultivating any one point will foster the cultivation of the others. If we get down to brass tacks, we are actually cultivating something deeper than all of these overtly superficial attitudes, and once we grasp that deeper understanding we can release the apparent rigidities of MN 39. But pedagogically speaking the format of MN 39 — a simple rhetorical question posed, an answer given, a repeated refrain declaring the rightness of the answer — is wonderfully effective for inculcating the basics in more-or-less illiterate populations..

Many of these things are certainly possible to do without cultivating the others.

You say

I am not sure how the step of purifying conduct and livelihood in the way he describes it does not the imply restraint of the senses from all the way up to the abandoning of the hinderances

And then

Same with restraint of the senses until the abandoning of the hinderances and mindfulness and full awareness until the latter.

It is not entirely clear what you are saying, but it seems like you are asking something along the lines of:

How is it possible to purify conduct and livelihood without simultaneously restraining the senses, being moderate in eating, being devoted to wakefulness, and being devoted to mindfulness and alertness? Similarly how is it possible to restrain the senses without being moderate in eating, being devoted to wakefulness, being devoted to mindfulness and alertness, and being devoted to abandoning the hindrances?

Perhaps the best way to understand this is to think of each step as describing relative degree of success. For example, plenty of people live ethical lives, in other words they have “purified conduct and livelihood,” without being committed to sensual moderation and/or high level mindfulness. They may be kind, generous, and harmless to others, but still eat more than is healthy, consume entertainment, and take regular naps. A step up from this may be a person who is similar in external behavior, but they also do a better job of watching over their minds reactions to life’s ups and downs so they spend less time daydreaming about fantasies or hating/fearing pain. A step up from this would be a person who is similar, but is moderate in their eating. And so on.

Of course, this doesn’t mean that cultivating the “higher steps” will have no effect on the “lower steps.” For example, being moderate in eating may help with restraint of the senses. Practicing mindfulness may help with getting to an even higher degree of purified mental conduct.

Just like in learning any new skill, you start with the relatively more basic steps first, then progress on to the more advanced subskills. However, this doesn’t mean that you are done mastering the more basic subskills.