Iām afraid there is nothing simple about it. Attempts so far have failed to show that there is a consistent logic behind the usage of manas, citta, and vinnana in the suttas - sometimes they mean similar phenonema, sometimes different, whereas citta and vinnana seem closer to each other than manas.
I think there is a similar ambiguity in the usage of the English words/concepts mind and consciousness except that we have the advantage of modern philosophy, linguistics, etc to hone in on more defined usable meanings. With Pali we are dealing with a dead thought world.
But all Iām doing is playing at apologetics by showing that there is leeway to chuck pretty much anything I want into the range of manayatana since dhamma is such a potentially broad category. In other words, it is perfectly conceivable that anything that can be experienced that does not partake of the first 5 sensory-fields would have been included under the sensory-field of the mind by the Buddha. It isnāt necessarily the case that this is so but it might still be the case.
Besides, there is one sutta I can cite which suggests that if the discourses on the 6 sense spheres and those on the arupas are not coming from two different schools of thought or one muddled thinker then the arupas must fall under the domain of manayatana.
At Savatthi. āBhikkhus, I will teach you the all. Listen to thatā¦.
āAnd what, bhikkhus, is the all? The eye and forms, the ear and sounds, the nose and odours, the tongue and tastes, the body and tactile objects, the mind and mental phenomena. This is called the all.
āIf anyone, bhikkhus, should speak thus: āHaving rejected this all, I shall make known another allāāthat would be a mere empty boast on his part. If he were questioned he would not be able to reply and, further, he would meet with vexation. For what reason? Because, bhikkhus, that would not be within his domain.ā - SN 35.23
Somaratne writes here (p.185) that manas and vinnana are being āperfumed by cittaā - as maddeningly poetic this is where weād hope for clarity Iām afraid this really gets as concrete as the suttas are.
I tend to see the suttas as being at times pretty loosey-goosey with terminology. Presumably because that terminology was already being used in a rough, perhaps semi-nebulous, manner, or because the Buddha or the compilers didnāt see any need to be too analytical or rigorous in defining their terms, or both.
I donāt pretend to have a solution for that (because the suttas donāt), but understanding ayatana as locus of experience we cannot assume either that ayatanas are like babushkas: an ayatana within an ayatana within an ayatana. Nowhere do the suttas say that the endless-space-ayatana is within manayatana. A more simple reading (to me at least) is that leaving the salayatana results in reaching the ÄkÄsÄnaƱcÄyatana.
Iām not saying their is some babushka hierarchy of ayatanas going on, just that words can be used in different ways. If I said that a football field was in my field of vision that would make perfect sense but I wouldnāt usually speak that way. Instead Iād say I was at a football field and it would be implied that the football field was in my visual field, assuming the person I was talking to knew I wasnāt blind. I can imagine that it is similarly implied that if someone says they had been dwelling in ÄkÄsÄnaƱcÄyatana that they were having a kind of purely mental experience, that they were in contact with some dhamma via manayatana.
And again Iād refer to the sutta above to suggest that the simpler reading is to include ÄkÄsÄnaƱcÄyatana within manayatana unless we assume that the suttas on the arupas and those on the six sense spheres come from different authors or one muddle-minded author.
- the upanishads for example use the much clearer saį¹kalpa, i.e. saį¹
kappa (e.g. in BU 1.5.3, BU 2.4.11, BU 4.5.12) ā¦