We would need to see the original Pali to know for sure how this sutta has been managed in translation.
Here is the sutta, MN 36.3, in its original Pali:
“Tisso imā, bhikkhave, vedanā. Katamā tisso? Sukhā vedanā, dukkhā vedanā, adukkhamasukhā vedanā. Sukhāya, bhikkhave, vedanāya rāgānusayo pahātabbo, dukkhāya vedanāya paṭighānusayo pahātabbo, adukkhamasukhāya vedanāya avijjānusayo pahātabbo. Yato kho, bhikkhave, bhikkhuno sukhāya vedanāya rāgānusayo pahīno hoti, dukkhāya vedanāya paṭighānusayo pahīno hoti, adukkhamasukhāya vedanāya avijjānusayo pahīno hoti, ayaṃ vuccati, bhikkhave, ‘bhikkhu niranusayo sammaddaso acchecchi taṇhaṃ, vivattayi saṃyojanaṃ, sammā mānābhisamayā antamakāsi dukkhassā’ti.
Sukhaṃ vedayamānassa,
vedanaṃ appajānato;
So rāgānusayo hoti,
anissaraṇadassino.
Dukkhaṃ vedayamānassa,
vedanaṃ appajānato;
Paṭighānusayo hoti,
anissaraṇadassino.
Adukkhamasukhaṃ santaṃ,
bhūripaññena desitaṃ;
Tañcāpi abhinandati,
neva dukkhā pamuccati.
Yato ca bhikkhu ātāpī,
sampajaññaṃ na riñcati;
Tato so vedanā sabbā,
parijānāti paṇḍito.
So vedanā pariññāya,
diṭṭhe dhamme anāsavo;
Kāyassa bhedā dhammaṭṭho,
saṅkhyaṃ nopeti vedagū”ti.
Tatiyaṃ.
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Oh, and by the way, my Pali vocabulary is some dozen words or so, which is almost as much as my English. But here's a stab at translation of the above in bold:
adukkha-m-asukhá vedaná: it literally means 'a feeling which is neither painful nor joyful'. It is commonly interpreted as an indifferent feeling; however this term appears in the Buddha's description of the 4th stage of meditative absorption (jhana) and thus is most likely the sensory characteristics that are associated with meditative absorption (jhana-nimitta), which he said were not originating from sensory stimulation. The gnostic term would be 'charism.'khandha, vedaná. (D. 22)[source: GWV Dictionary]
avijjānusayo: the flood of ignorance, the yoke of ignorance, the tendency to ignorance [source: Ancient Buddhist Texts.net; couldn't find in GWV Dictionary]
There are other suttas in the MN and DN that have referred to three types of feeling. There I found the neither-painful-nor-pleasant feelings were jhana. Here are some other quotes that may explain this issue with clarity:
I missed these suttas' references to neither-painful-nor-pleasant feeling when I first read through them. But at the time of reading I wasn't looking specifically for them. It shows that one must reread the suttas at depth to appreciate the details. But I'm so happy that you have provided us with this group of suttas. It clarifies what neither-painful-nor-pleasant feelings are. I will add these suttas to my notes on feelings.
Bahuvedaniya Sutta, MN 59
.3 "This neither-painful-nor-pleasant feeling has been stated by the Blessed One as a peaceful and sublime kind of pleasure."
.7 "Now, if someone were to say: 'This is the highest pleasure and joy (sense pleasure) that can be experienced,' I would not concede that. And why not? Because there is another kind of pleasure (neither-painful-nor-pleasant) which surpasses that pleasure and is more sublime. And what is this pleasure? Here, quite secluded from sensual desires, secluded from unwholesome states of mind, a contemplative enters upon and abides in the first meditative absorption (jhana) ... and has in it joy and pleasure born of seclusion. This is the other kind of pleasure which surpasses that (sense) pleasure and is more sublime."
Based upon a translation of the Majjhima Nikaya trans. Bhikkhus Nanamoli & Bodhi, Wisdom, 1995, Edited by Jhananda
I especially appreciate this sutta since the neither-painful-nor-pleasant feelings refer obviously to jhana.
So we have two physical feelings: one pleasant, and the other painful. They are either emotional or of the six senses. And one feeling, "neither-painful-nor-pleasant" which is of a spiritual-nonsensual-pleasant nature.
It is important that we understand these three feelings since they are the triggers for the Three Unwholesome Roots: greed, hatred and delusion. And it is the goal of the N8P to overcome these three defilements.