Well, before I answer that question, we should revisit MN 119 and MA 81. I’m kind of surprised you didn’t correct me when I combined the refrain that’s found at the end of each of the mindfulness exercises in MN 119 with the fourth jhana. If you’ve read the sutta, you’ll know that it contains exercises like contemplating all the parts of one’s body, being aware of one’s surroundings so as not to be clumsy, and so on. Each of those exercises end with that same refrain:
Tesaṁ pahānā ajjhattameva cittaṁ santiṭṭhati sannisīdati ekodi hoti samādhiyati.
Their mind becomes stilled internally; it settles, unifies, and becomes immersed in samādhi.
It really doesn’t have anything to do with the fourth jhana per se, other than the jhanas were considered a type of mindfulness exercise. I didn’t recall this in the morning when I replied to you; I assumed it was because the phrase about inner peace occurs in some dhyana descriptions in Chinese, and I’m getting old. This stuff blurs together at a certain point.
But let’s look at what MA 81 says instead, because it’s different.
如是比丘隨其身行。便知上如眞。彼若如是在遠離獨住。心無放逸修行精勤。斷心諸患而得定心。
“Thus, a monk follows his physical actions, knowing the above as it truly is. If he thus goes into seclusion and lives alone, his thoughts aren’t careless. He cultivates diligence, stops mental disturbances, and attains a concentrated state of mind.
This is repeated after each of the mindfulness exercises. It’s more explicit about a monk going into seclusion and practicing meditation. It doesn’t use the “internal stillness” language, but instead says that disturbances of the mind are stopped (斷心諸患), and then he attains a settled or concentrated mind (得定心).
Is this meaning different than what we read in MN 119? Yes, literally it is different, but I think stillness in Pali doesn’t mean refer to something different than ending disturbances in Chinese. They are two ways of describing the same thing.
So, there’s that.
All I mean with the pond metaphor is that the mind isn’t disturbed. The water is settled. The water would be one’s mind. The waves would be the emotional disturbances that are said to stop in the fourth jhana. In the third jhana, the problem is attachment to the happiness experienced in it. That has to be let go to reach the fourth jhana. So, the equanimity of the fourth jhana is one that isn’t bothered anymore by desire or dislike caused by pleasant or painful feelings. It’s not that the meditator ceases to think; rather, they cease to be derailed by the pleasure they feel while in samadhi. With pure equanimity, they can think clearly and understand correctly. Then, when they contemplate subjects like dependent origination or the aggregates, they can penetrate the realities those teachings describe.
That is the way I see it at this point after reading a lot of the commentaries that exist on the subject. Contemplating topics like dependent origination isn’t included in the Abhidharma commentaries, but they are mentioned in sutras. In many cases, samadhi isn’t the end goal of practice, rather it’s the way to develop wisdom, so it’s the next to the last step of practice.
There are types of meditation that sound very similar to what you are associating with the jhanas. The formless samadhis or the three samadhis of emptiness, signlessness, and wishlessness sound more like what you are thinking about. And there’s the samadhi of cessation, of course, in which conceptions (or perceptions) finally cease. Abhidharma texts contain debates about those kinds of samadhi, asking questions like “How is it different than being unconscious? How does the meditator emerge from samadhi if they have no mental activity?” So, those were definitely considered to be states of mind without mental activity. But they don’t have discussions like that about the fourth jhana.