In Tibetan Buddhism, often the so-called 'clear-light mind' is mentioned. This is the most subtle level of mind (see also death & rebirth), which we are normally not even aware of. It appears to the very advanced meditator and during the death process, but in this case, also only advanced meditators will be able to notice it. It is a non-conceptual, 'primordial' state of mind.
From a talk given by HH Dalai Lama. Oct. 11-14, 1991 New York City. Path of Compassion teaching preliminary to Kalachakra:
Question: When people hear of luminosity of clear light that dawns at the moment of death they ask why it is called clear light. What has this got to do with light as we know it?
"I don't think that in the term clear light, light should be taken literally. It is sort of metaphoric. This could have its roots in our terminology of mental will. According to Buddhism, all consciousness or all cognitive mental events are said to be in the nature of clarity and luminosity. So it is from that point of view that the choice of the term light is used. Clear light is the most subtle level of mind, which can be seen as the basis or the source from which eventual experience or realisation of Buddhahood, Buddha's wisdom might come about, therefore it is called clear light. Clear light is a state of mind which becomes fully manifest only as a consequence of certain sequences or stages of dissolution, where the mind becomes devoid of certain types of obscurations, which are again metaphorically described in terms of sun-like, moonlike and darkness. These refer to the earlier three stages of dissolution which are technically called, including the clear light stage, the four empties. At the final stage of dissolution the mind is totally free of all these factors of obscuration. Therefore it is called clear light. Sort of a light. It is also possible to understand the usage of the term clear light in terms of the nature of mind itself. Mind or consciousness is a phenomena which lacks any obstructive quality. It is non-obstructed."
A teaching from Venerable Ajahn Chah (Pra Bhodinyana Thera):
"About this mind... In truth there is nothing really wrong with it. It is intrinsically pure. Within itself it's already peaceful. That the mind is not peaceful these days is because it follows moods. The real mind doesn't have anything to it, it is simply (an aspect of) Nature. It becomes peaceful or agitated because moods deceive it. The untrained mind is stupid. Sense impressions come and trick it into happiness, suffering, gladness and sorrow, but the mind's true nature is none of those things. That gladness or sadness is not the mind, but only a mood coming to deceive us. The untrained mind gets lost and follows these things, it forgets itself. Then we think that it is we who are upset or at ease or whatever.
But really this mind of ours is already unmoving and peaceful... really peaceful! Just like a leaf which is still as long as no wind blows. If a wind comes up the leaf flutters. The fluttering is due to the wind -- the 'fluttering' is due to those sense impressions; the mind follows them. If it doesn't follow them, it doesn't 'flutter.' If we know fully the true nature of sense impressions we will be unmoved.
Our practice is simply to see the Original Mind. So we must train the mind to know those sense impressions, and not get lost in them. To make it peaceful. Just this is the aim of all this difficult practice we put ourselves through."