Since Ben is busy - i will improvise some comedic relief while we wait...
This stuff is soooo fascinating - i am in no way a scholar but have a little basic understanding - i am "flying by the seat of my pants" here - so please correct me. (My basic knowledge came from "The Comprehensive Manual of Abhidhamma" - Ed. Bodhi and "Unravelling the Mysteries of Mind & Body through Abhidhamma" - Sayalay Susila)
Great questions - Why one citta
at a time? What is the proximate cause for the cessation of a citta
We are discussing merely the awareness of an object - not comprehension by knowledge or wisdom - that is macro level - at the micro level of citta
- a photon of light enters the eye - a molecule of aroma enters the nose, etc. While it may seem as if more than one photon can enter at a time, or one molecule of odor - whatever - when it gets funneled into the pipeline for processing it is 'one at a time'.
Hmmm... perhaps this is a cop-out - but "one at a time" because this is the subhava
or 'intrinsic nature' of this ultimate reality, our consciousness. Every citta has an object and it cannot accomodate more than one object. This does not explain 'why' - but just like a movie appears as continuous motion on the movie screen - the intrinsic nature is a rigid moment framed and separate from the next. 2500 plus years ago the monks were not only 'seeing' each frame - but actually walking around and looking at the borders of each frame.
Many, many, many every second. Amazing. Quantum by defintion.
It is like the Mandelbrot series in mathematics - understanding the pattern at the large scale we find the same patterns repeating at each smaller scale. At a macro level we live only one life at a time - at the micro level we live only one moment of consciousness at a time. And just as each lifetime has its own birth, aging and death - so too each moment of consciousness has its birth, aging and death. (The spelling 'ageing' looks better to me - i must have been british in a past life
Which leads to the next question of proximate cause - (i hope you mean primary cause)...
if you think about it, it is simple - in this lifetime - what is the cause of your death? - your birth! Again the same things that apply at the large scale of a life time apply at the tiniest scale of a moment of time - the citta
If you didn't mean primary - well, you'd have to specify exactly which citta you wish to examine - differing types of citta have differing life spans - as per humans - and different reasons for death - but primarily - birth causes death.
When you look at the orderly diagrams of "this followed by that" at the citta
level it looks like the "Paticca-samuppada-vibhanga Sutta: Analysis of Dependent Co-arising"
where you find the complete cycle of "this leads to that followed by this followed by that... "
So, in summary - for the same reason that we live our many many lives in a sequential 'one at a time' orderliness - we experience each of our many many moments of consciousness in a sequential 'one at a time' orderliness. The proximate cause for the the dissolution of a citta is its arising.
Hopefully you've enjoyed a moment of brevity - such is my tap dance here - but i'll close with some interesting excerpts from page 134-6 of "Unravelling" regarding the controversy of "How separated is realizing the 'path' of stream-entry' to realizing the 'fruit of stream-entry'?":
When one's insight knowledge becomes matured ... one attains Nibbana. At that time, the cognitive process of supramundane stream-entry path (sotipatti magga vithi) runs as follows:
First, ... <skipping a lot> ... then the change-of-lineage (gotrabhu) arises which takes as its object the signlesss, the non-occurence, the cessation, Nibbana, which passes out of the lineage of worldlings (puthujjana) and enters into the lineage of the Noble Ones (Ariya). It gives the sign to the Path to come into being, and ceases.
Without pausing, the Path follows in uninterrupted continuity, piercing and exploding the mass of greed, hatred and delusion, never pierced nor exploded before. Path consciousness arises only once, taking Nibbana as its object. After Path consciousness, without any delay in time, there arise two or three moments of Fruition consciousness, which are its result, taking Nibbana as its object. At this point, one becomes a stream-enterer...
Although change-of-lineage takes Nibbana as object, it does not perform the function of eliminating defilements as does Path consciousness. Path consciousness arises for only one moment then passes away. The same type of Path consciousness never arises again for a second time... Stream-entry path consciousness permanently uproots three fetters:
... sakkaya-ditthi ... silabbataparamasa ... vicikiccha ...
i assume the 'one attains Nibbana' to refer to that first glimpse of 'the deathless' that stream-entry gives. According to this the fruition follows right on the heels of the path - but they are separate if only for a conscious moment...