This is a big article but worth the read:
http://www.alanwallace.org/Vacuum%20States%20Essay.pdfHere's some snips in what he said:
"The cognitive basis of all mental activities and sensory perceptions is the
bhavaºga, literally, the ground of becoming, which supports all kinds of javana, as
the root of a tree sustains the trunk, branches, and leaves.
This is the resting, ground state of consciousness, withdrawn from the physical senses. While all mental and sensory processes are conditioned by the body and the environment, in the Buddhist view they actually emerge from the bhavaºga, not the brain.
Described as the natural, unencumbered state of the mind, its innate radiance
and purity are present even when the mind is obscured by afflictive thoughts
and emotions. The bhavaºga may be characterized as a “vacuum” state of
consciousness, voided of all manner of javana. Generally speaking, it is
indiscernible while the mind is active, for it normally manifests only in
dreamless sleep and during the very last moment of a person’s life. Indeed,
Buddha declared that there are multiple similarities in the cognitive processes
while falling asleep and dying.
To unlock this natural purity and luminosity of consciousness so that its
radiant potential is revealed, one must calm the involuntary activity of the mind
through the practice of meditative quiescence.5 In this way, one can see through
the superficial turbulence of the mind into its limpid depths. In the Buddhist
view, the bhavaºga acts as the basis for all volitional states of consciousness, and
thus for karma; and it is therefore the basis for the emergence of the world
experienced by each individual.
The world each of us experiences does not consist of an independent
subject observing independent objective phenomena. Rather, the various modes
of sensory and mental consciousness arise from moment to moment in relation to
the phenomena that appear to them and in dependence upon those cognitive
faculties. The duality of subject and object is something conceptually
superimposed on experience, not something that is discovered through empirical
observation. So without resorting to the stance of philosophical idealism, it may
be said that the bhavaºga serves as the nondual source of creation of each
person’s experienced-world-and-its-experiencer.
The above account of javana and the bhavaºga is based on the Buddha’s
discourses recorded in the P›li language and their earliest commentaries. A
remarkably similar description of the ground state of consciousness appears in
the later Great Perfection (Dzogchen) school of Tibetan Buddhism. Here a
distinction is made between the substrate (›laya) and the substrate consciousness
(›layavijñ›na). Tibetan contemplatives describe the substrate as the objective,
empty space of the mind. This vacuum state is immaterial like space, a blank,
unthinking void into which all objective appearances of the physical senses and
5
mental perception dissolve when one falls asleep; and it is out of this vacuum
that appearances re-emerge upon waking.
The subjective consciousness of this mental vacuum is called the substrate
consciousness. In the natural course of a life, this is repeatedly experienced in
dreamless sleep and finally experienced in the moment before death. A
contemplative may consciously probe this dimension of consciousness through
the practice of meditative quiescence, in which discursive thoughts become
dormant and all appearances of oneself, others, one’s body and one’s
environment vanish. At this point, as in the cases of sleeping and dying, the
mind is drawn inwards and the physical senses become dormant. What remains
is a state of radiant, clear consciousness that is the basis for the emergence of all
appearances to an individual’s mind-stream. All phenomena appearing to
sensory and mental perception are imbued with the clarity of this substrate
consciousness. Like the reflections of the planets and stars in a pool of limpid,
clear water, so do the appearances of the entire phenomenal world appear within
this empty, clear substrate consciousness. Contemplatives who have penetrated
to this state of consciousness describe it as “an unfluctuating state, in which one
experiences bliss like the warmth of a fire, luminosity like the dawn, and
nonconceptuality like an ocean unmoved by waves.”
The above description can easily be misinterpreted as an expression of
philosophical idealism. However, these contemplatives are not claiming that the
entire universe is of the nature of the mind, only that one’s individual world of
appearances arises from this substrate consciousness. Moreover, the qualities of
bliss, luminosity, and nonconceptuality associated with the realization of the
substrate consciousness have led many contemplatives to mistake this for the
ultimate nature of reality, or nirv›°a. But simply dwelling in this relative vacuum
state of consciousness does not liberate the mind of its afflictive tendencies or
their resultant suffering. By fathoming the nature of the substrate consciousness,
one comes to know the nature of consciousness in its relative ground state. This
realization, however, does not illuminate the nature of reality as a whole. It is
also important not to confuse this substrate consciousness with a collective
unconscious, as conceived by Carl Jung. Buddhist accounts of the substrate
consciousness all refer to it as an individual stream of consciousness that carries
on from one lifetime to the next."
and
"The experiential realization of absolute space by primordial consciousness
transcends all distinctions of subject and object, mind and matter, indeed, all
words and concepts. Such insight does not entail the meeting of a subjective
mode of consciousness with an objective space, but rather the nondual
realization of the intrinsic unity of absolute space and primordial consciousness.
The dharmadh›tu and primordial consciousness are coterminous, nonlocal, and
atemporal. While the dharmadh›tu is the fundamental nature of the experienced
world, primordial consciousness is the fundamental nature of the mind. But
since the two have always been of the same nature, the view of the Great
Perfection is not one of philosophical idealism, dualism, or materialism. All such
distinctions between subject and object, mind and matter are regarded as mere
conceptual fabrications."
Rest of article at link.
