All those arupa jhana are refered to as a base, and while one is in it, one perceives this base (MN1)
i see it like this that these bases are never ever absent. But are bases of the mind. Levels, as it were.
Like making a descent into a building and arriving at the most deepest base.
Jhana is like making a conscious descent into the most subtle levels of the mind.
Cessation is the most subtle and deepest base, i believe. One arrives at the nature of the mind.
These bases also can be wrongly conceived as me, mine, myself.
What is refered to here as base means, i believe, that it is never ever sensed as some formation like greed, hate, piti etc. that is coming and going. It is like the sky is the base for clouds to arise in. Likewise with arupa jhana. The base refers to the ground as it were.
One can find a deeper base, an even more subtle ground in arupa jhana, but this is never seen as a formation coming and going. It is only a more refined, even more subtle base.
In the end one arrives at the mind base itself, very subtle, hard to see, empty of perceptions and feeling.
One never becomes mindless but one finally sees the most deep, subtle nature of mind.
i feel, this whole progressive stilling comes down to finding the most subtle base of the mind. The subtle, the very hard to see, the stable, constant, the unconditioned.
But, this is never absent, but during jhana one makes a conscious descent into what is never absent.
All jhana are perceived as peaceful abidings and as such known. Not in a retrospective way but while abiding in it. Also nothingness.