Interesting discussion and thank you to all for it. I was prompted to register so that I could add a comment and offer a reference for your consideration. I don't subscribe to the Alayavijnana thinking in any form. To be honest my view has its basis in meditative experience but I understand this to be an academic forum so I will restrict my comment to a reference and some related reasoning in the hope it might stimulate some 'academic' interests.
Here is the reference:
MN 106 Aneñja-sappaya Sutta: Conducive to the Imperturbable
http://www.accesstoinsight.org/tipitaka ... .than.html
I see this discourse as related to the discussion. It is best to read the entire short sutta but I will copy here below the more relevant portion from the end of the discourse.
My thinking is that the fundamental mental support for all forms of consciousness is the same as that which alone remains in the formless concentration commonly referred to in the pali suttas as
neither perception nor non-perception. I think that in the
cessation of perception and feeling or
nirodha samapatti even this subtlest possible form of consciousness is also abandoned and with it all samsaric phenomena.
My thinking on why consciousness resumes is more complicated as the resumption of consciousness will depend on the extent of clinging remaining in the one who has entered into the cessation. In simplest terms consciousness resumes because consciousness' continual re-arising and it's clinging to it's persistent being is the fundamental ignorance and therefore also phenomenologically fundamental to all more complex compounded reflexive kamma making phenomena.
In the worlding consciousness is untrained and unskilled and can do nothing else. As in MN 1 where the worlding is referred to as conceiving even in regards to formless states and in regards to nibbana.
In noble persons who are well trained, disciplined and accomplished there is some development of understanding and comprehension and as a consequence various forms of dependance and types of kamma making has been abandoned to one extent or another.
In fully liberated noble persons the neither perceiving nor non-perceiving consciousness has been examined and comprehended to be that subtlest type of conscious samsaric phenomena which clings to its re-arisings and that which seeks out diverse perceptual contacts and thereby by means of dependent compounding of phenomena diverse sensations, forms and all the rest. In those for whom ignorance has ended, in whom all craving, aversion and clinging has been fully abandoned, all resort to this
supreme sustenance has likewise been rightly understood for what it is and rightly abandoned as it is at the root of all suffering.
So for the fully liberated there is no involvement in kamma making at the root through dependence on consciousness only resultant consciousness kamma which is enough to support being but is insufficient to serve for ongoing becoming in any future form. In the arahant the root has been cut and I take that root to be dependence on consciousness of any sort and the cutting of it to be the realizing and understanding of its nature, modes and functions and the resulting dispassion with that process and abandoning of that process.
I would think should a fully liberated one so wish they need not return from nirodha samapatti and could simply allow the body to eventually expire while continuing to abide in that way and thereby enter into parinibbana. All other noble persons, those not yet fully understanding and fully liberated would have to continue to work to extinguish all basis for samsaric dependence including dependence on formless states before they are likewise fully liberated and so even if they can enter into cessation for a time any other form of dependence or basis for consciousness would be cause for the resumption of consciousness and for their ongoing being and becoming.
I hope that has been straightforward enough to be understandable. I don't see the need for positing any other kind of consciousness which could be said to persist during nirodha samapatti or for positing an unconscious consciousness. I think all of the necessary basis for the resumption of consciousness are already known and given in the pali sutta discourses and the causes for the re-arising of consciousness, while multiple and various are not at all inexplicable or unknowable or any other than those which are the common causes for the ongoing re-arising of consciousness in all beings.
"Then again, the disciple of the noble ones considers this: 'Sensuality here & now; sensuality in lives to come; sensual perceptions here & now; sensual perceptions in lives to come; forms here & now; forms in lives to come; form-perceptions here & now; form-perceptions in lives to come; perceptions of the imperturbable; perceptions of the dimension of nothingness: all are perceptions. Where they cease without remainder: that is peaceful, that is exquisite, i.e., the dimension of neither perception nor non-perception.' Practicing & frequently abiding in this way, his mind acquires confidence in that dimension. There being full confidence, he either attains the dimension of neither perception nor non-perception now or else is committed to discernment. With the break-up of the body, after death, it's possible that this leading-on consciousness of his will go to the dimension of neither perception nor non-perception. This is declared to be the practice conducive to the dimension of neither perception nor non-perception.
When this was said, Ven. Ananda said to the Blessed One: "There is the case, lord, where a monk, having practiced in this way — 'It should not be, it should not occur to me;[2] it will not be, it will not occur to me.[3] What is, what has come to be, that I abandon' — obtains equanimity. Now, would this monk be totally unbound, or not?"
"A certain such monk might, Ananda, and another might not.'
"What is the cause, what is the reason, whereby one might and another might not?"
"There is the case, Ananda, where a monk, having practiced in this way — (thinking) 'It should not be, it should not occur to me; it will not be, it will not occur to me. What is, what has come to be, that I abandon' — obtains equanimity. He relishes that equanimity, welcomes it, remains fastened to it. As he relishes that equanimity, welcomes it, remains fastened to it, his consciousness is dependent on it, is sustained by it (clings to it). With clinging/sustenance, Ananda, a monk is not totally unbound."
"Being sustained, where is that monk sustained?"
"The dimension of neither perception nor non-perception."
"Then, indeed, being sustained, he is sustained by the supreme sustenance."
"Being sustained, Ananda, he is sustained by the supreme sustenance; for this — the dimension of neither perception nor non-perception — is the supreme sustenance. There is [however] the case where a monk, having practiced in this way — 'It should not be, it should not occur to me; it will not be, it will not occur to me. What is, what has come to be, that I abandon' — obtains equanimity. He does not relish that equanimity, does not welcome it, does not remain fastened to it. As he does not relish that equanimity, does not welcome it, does not remain fastened to it, his consciousness is not dependent on it, is not sustained by it (does not cling to it). Without clinging/sustenance, Ananda, a monk is totally unbound."
"It's amazing, lord. It's astounding. For truly, the Blessed One has declared to us the way to cross over the flood by going from one support to the next. But what is the noble liberation?"
"There is the case, Ananda, where a disciple of the noble ones considers this: 'Sensuality here & now; sensuality in lives to come; sensual perceptions here & now; sensual perceptions in lives to come; forms here & now; forms in lives to come; form-perceptions here & now; form-perceptions in lives to come; perceptions of the imperturbable; perceptions of the dimension of nothingness; perceptions of the dimension of neither perception nor non-perception: that is an identity, to the extent that there is an identity. This is deathless: the liberation of the mind through lack of clinging/sustenance.'
"Now, Ananda, I have taught the practice conducive to the imperturbable. I have taught the practice conducive to the dimension of nothingness. I have taught the practice conducive to the dimension of neither perception nor non-perception. I have taught the way to cross over the flood by going from one support to the next, the noble liberation. Whatever a teacher should do — seeking the welfare of his disciples, out of sympathy for them — that have I done for you. Over there are the roots of trees; over there, empty dwellings. Practice jhana, Ananda. Don't be heedless. Don't later fall into regret. This is our message to you all."
That is what the Blessed One said. Gratified, Ven. Ananda delighted in the Blessed One's words."