Verified answer

My guess is:

The objects of awareness for akusala vipaka cittas are usually unpleasant arammanas (for example, bad smell, unpleasant taste, disgusting sight) that are the results of past unwholesome actions (akusala kamma). So the mind that investigates those unpleasant objects or santirana citta probably cannot have pleasant feeling. That is why akusala vipaka cittas do not have somanassa feeling.

The objects for kusala vipaka cittas or the resultant minds of past wholesome actions are usually pleasant ones. The investigating mind or santirana citta can have either the feeling of equanimity or the joyful feeling towards the pleasant object. I think that is the reason why there is an additional kusala vipaka santirana citta with somanassa feeling.

quote: 'there are 7 ahetuka akusala vipaka cittas and 8 ahetuka kusala vipaka

cittas totalling in 15 ahetuka vipaka cittas.' p18

8.somanassa sahagatam ahetuka kusala santirana citta

http://sanskritdocuments.org/articles/Notes_on_PAT...

Read the glossary as well.

The term is also used a) for the classification of consciousness, as sa-hetuka and a-hetuka with and without concomitant root-conditions; b) for a division of rebirth consciousness into ahetuka dvihetuka and tihetuka without, with 2, or with 3 root-conditions see: patisandhi

http://what-buddha-said.net/library/Buddhist.Dicti...

if rebirth consciousness is with painful existence, why doesn't domanassa occur as the 8th ahetuka akusala citta?

perhaps the opposite of joy doesn't exist. perhaps it's joy or no joy, that's all. in bad rebirth, there is no joy.

To combine such words (somanassa ahetuka akusala...) might be impossible or meaningless.

VERY INTERESTING

but idk

edit:

read this one too

"If one is born with somanassa, happy feeling, all bhavanga-cittas of that life are accompanied by somanassa."

http://www.midamericadharma.org/gangessangha/Bhava...