Which aggregate (khandha) is or generates 'suffering'?

SN 56.11 says upādānakkhandha explained here as follows:

Here a being is viewed as the collection of five aggregates of phenomena, to which untrained beings are deeply attached out of ignorance, considering them - without discerning them - as constituents of their attā, which is described as the ultimate nature of dukkha by the formula 'saṃkhittena pañcupādānakkhandhā dukkhā': see for example SN 56.11.

They are defined at SN 22.79. It is stated at MN 43 that vedanā, Saññā and Viññāṇa are deeply associated and that it is impossible to clearly separate one from another to show their difference.

The bit of MN 43 which is quoted by the above is,

• Friend, as to Vedanā Saññā and Viññāṇa: are these dhammas conjoined or disjoined? And is it possible, having separated them one from another to point out the difference between them?

• Friend, as to Vedanā Saññā and Viññāṇa: these dhammas are conjoined, not disjoined. It is not possible, having separated them one from another, to point out the difference between them. For whatever one feels, friend, that one perceives, and whatever one perceives, that one cognizes. Therefore these dhammas are conjoined, not disjoined. It is not possible, having separated them one from another, to point out the difference between them.

A precise answer can be found in Samyutta Nikaya, Khandha Samyutta, Bhära vagga - Asamula Sutta

The aggregates(panchakkandha) are: Rupa, Vedanä, Sangna, Sankhara, Vinñana.

By definition, the liking (upadäna) toward these aggregates (panchakkandha) is suffering (panchaupädänakkhanda).

This last sentence means,

Concisely, the grasping of aggregates, is suffering.

Grasping is more of being ensnared.

So, what causes suffering? Birth, i.e. The formation of eyes, ears, nose, tongue, body and mind. What causes birth? Conditioning of karma for fruition. What causes conditioning of karma for fruition? Craving. What causes craving? Feeling. What causes feeling? Contact. What causes contact? The six senses. What causes the six senses? Five Aggregates. What causes the five aggregates? Vinñana. What causes Vinñana? Consecutive activities. What causes constructive activities? Incomprehension (Ignorance is barely a fit to describe avijja)

I don't have the complete meditative experience or knowledge of the Dhamma to be able to fully comprehend the intents behind the teachings of the Five Aggregates and Twelve Nidanas, but I think I have some insights. I suspect that they are observable abstracted analysis of the complex working of the mind rather than actual entities within the mind itself - just as the body is only labeled 'form', but can be further analyzed in it's components such as the organs, or states of solidity, liquidity, gas and heat (earth, water, wind, and fire in ancient terminology). In this sense I believe the mental aggregates overlap in functions.

For example, the mind and mental activities in normal daily linguistic usage can be divided into feelings/sensation, thoughts, emotions, memories. Psychologists can probably further divide them according to various theories.

So Vedanā - commonly translated as sensations, is the observable feeling of pleasant, unpleasant, neutral when our internal sense organs come into contact with external sense objects and the associated consciousness.

Sañña - typically translated as "perception" or "cognition." Interestingly in Chinese this is translated as 想 - "thoughts". So this is our thinking mind

Saṅkhāra - 'volitional formations' mental formation etc. I think this generally the emotional response. The Chinese translation 行 has a meaning of 'action'/'execution'.

And finally the Viññāṇa 'consciousness' seems to be the catch all bucket for the general 'state' of the mind. You can have a pure consciousness, defiled consciousness and so forth.

As we can see, some functions of this overlaps, but the division help us focus our attention of where to comprehend things from.

In the Satipatthana. The four domains are: mindfulness of the body mindfulness of feelings or sensations mindfulness of mind or consciousness mindfulness of dhammās.

Satipaṭṭhāna Ānāpānasati

1. Contemplation of the body

   1. Breathing long (Knowing Breath)
   2. Breathing short (Knowing Breath)
   3. Experiencing the whole body
   4. Tranquillising the bodily activities

2. Contemplation of feelings

   5. Experiencing rapture
   6. Experiencing bliss
   7. Experiencing mental activities
   8. Tranquillising mental activities

3. Contemplation of the mind

   9. Experiencing the mind
   10. Gladdening the mind
   11. Centering the mind in samadhi
   12. Releasing the mind

4. Contemplation of Dhammas

   13. Contemplating impermanence
   14. Contemplating fading of lust
   15. Contemplating cessation
   16. Contemplating relinquishment

The last one contemplation of the Dhamma is essentially a contemplation of all phenomena and of Truth.

The division of the mind into the four aggregates allow you to focus and observe the phenomena of impermanence, suffering and selflessness within them.

This is useful because we commonly suffer due to attachment to our feelings, thoughts, emotions, memories, consciousness.

The Buddha reminds us that all the aggregates are impermanent and hence subject to cessation, and hence attachment to all of them is suffering. They are not self because we don't have full control of them. But a positive way to look at it would also be that each of them is caused and conditioned, and we can try to create good causes to have good results. For example having good thoughts and enjoying the pleasant emotions and sensations resulting from that, without being attached to the loss of those sensations when the conditions ends.