I recall the natural state of mind being referred to as luminous somewhere in Buddhist canonical texts. I've always been very taken with that imaginary. I'm sure it's quite a common image in the texts but can anyone give me one or two examples of it. Also is this concept/image linked to the notion of Buddha Nature in some way?
The following quotes describing the Luminous Mind are from the book "The Tibetan Book of Living and Dying" by Sogyal Rinpoche:
The Nature of The Mind
Nyoshul Khenpo Rinpoche said:
Profound and tranquil, free from complexity,
Uncompounded luminous clarity,
Beyond the mind of conceptual ideas;
This is the depth of the mind of the Victorious Ones.
In this there is not a thing to be removed,
Nor anything that needs to be added.
It is merely the immaculate
Looking naturally at itself.
-- The Tibetan Book of Living and Dying, p. 50
The Innermost Essence
You might ask: "If realizing the View is realizing the nature of mind, what then is the nature of mind like?" Imagine a sky, empty, spacious, and pure from the beginning;
its essence is like this. Imagine a sun, luminous, clear, unobstructed, and spontaneously present; its nature is like this. Imagine that sun shining out impartially on us and all things, penetrating all directions; its energy, which is the manifestation of compassion, is like this: nothing can obstruct it and it pervades everywhere.
-- The Tibetan Book of Living and Dying, p. 157
The Practices For Dying
More intrinsically, Amitabha is the limitless, luminous nature of our mind. At death the true nature of mind will manifest at the moment of the dawning of the Ground Luminosity, yet not all of us may have the familiarity with it to recognize it. How skillful and compassionate the buddhas are to have handed down to us a method for invoking the very embodiment of the luminosity in the radiant presence of Amitabha!
-- The Tibetan Book of Living and Dying, p. 236-237
As I have explained, in death all the components of our body and mind are stripped away and disintegrate. As the body dies, the senses and subtle elements dissolve, and this is followed by the death of the ordinary aspect of our mind, with all its negative emotions of anger, desire, and ignorance. Finally nothing remains to obscure our true nature, as every- thing that in life has clouded the enlightened mind has fallen away. And what is revealed is the primordial ground of our absolute nature, which is like a pure and cloudless sky.
This is called the dawning of the Ground Luminosity, or "Clear Light," where consciousness itself dissolves into the all- encompassing space of truth. The Tibetan Book of the Dead says of this moment:
The nature of everything is open, empty and naked like the sky Luminous emptiness, without center or circumference: the pure, naked Rigpa dawns.
Padmasambhava describes the luminosity:
This self-originated Clear Light, which from the very beginning was never bom,
Is the child of Rigpa, which is itself without any parents—how amazing!
This self-originated wisdom has not been created by anyone—how amazing!
It has never experienced birth and has nothing in it that could cause it to die—how amazing!
Although it is evidently visible, yet there is no one there who sees it—how amazing!
Although it has wandered through samsara, no harm has come to it—how amazing}.
Although it has seen buddhahood itself no good has come to it— how amazing!
Although it exists in everyone everywhere, it has gone unrecog- nized—how amazing!
And yet you go on hoping to attain some other fruit than this else- where—how amazing!
Even though it is the thing that is most essentially yours, you seek for it elsewhere—how amazing!
Why is it that this state is called "luminosity" or Clear Light? The masters have different ways of explaining this. Some say that it expresses the radiant clarity of the nature of mind, its total freedom from darkness or obscuration: "free from the darkness of unknowing and endowed with the abil- ity to cognize." Another master describes the luminosity or Clear Light as "a state of minimum distraction," because all the elements, senses, and sense-objects are dissolved.
What is important is not to confuse it with the physical light that we know, nor with the experiences of light that will unfold presently in the next bardo; the luminosity that arises at death is the natural radiance of the wisdom of our own Rigpa, "the uncompounded nature present throughout all of samsara and nirvana."
The dawning of the Ground Luminosity, or Clear Light, at the moment of death is the great opportunity for liberation. But it is essential to realize on what terms this opportunity is given. Some modern writers and researchers on death have underestimated the profundity of this moment. Because they have read and interpreted the Tibetan Book of the Dead without the benefit of the oral instructions and training that fully explain its sacred meaning, they have oversimplified it and jumped to quick conclusions.
One assumption they then make is that the dawning of the Ground Luminosity is enlighten- ment. We might all like to identify death with heaven or enlightenment; but more important than mere wishful think- ing is to know that only if we have really been introduced to the nature of our mind, our Rigpa, and only if we have estab- lished and stabilized it through meditation and integrated it into our life, does the moment of death offer a real opportu- nity for liberation.
Even though the Ground Luminosity presents itself natu- rally to us all, most of us are totally unprepared for its sheer immensity, the vast and subtle depth of its naked simplicity. The majority of us will simply have no means of recognizing it, because we have not made ourselves familiar with ways of recognizing it in life. What happens, then, is that we tend to react instinctively with all our past fears, habits, and condi- tioning, all our old reflexes. Though the negative emotions may have died for the luminosity to appear, the habits of lifetimes still remain, hidden in the background of our ordinary mind. Though all our confusion dies in death, instead of surrendering and opening to the luminosity, in our fear and ignorance we withdraw and instinctively hold onto our grasping.
This is what obstructs us from truly using this powerful moment as an opportunity for liberation. Padmasambhava says: "All beings have lived and died and been reborn count- less times. Over and over again they have experienced the indescribable Clear Light. But because they are obscured by the darkness of ignorance, they wander endlessly in a limitless samsara."
-- The Tibetan Book of Living and Dying, p. 263-265
The Universal Process
Remember now that when we looked at the nature of mind, we saw that it had these three same aspects: its empty, skylike essence, its radiant luminous nature, and its unob- structed, all-pervasive, compassionate energy, which are all simultaneously present and interpenetrating as one within the Rigpa. Padmasambhava describes this in the following way:
Within this Rigpa, the three kayas are inseparable and fully pres- ent as one:
Since it is empty and not created anywhere whatsoever, it is the Dharmakaya,
Since its luminous clarity represents the inherent transparent radi- ance of emptiness, it is the Sambhogakaya.
Since its arising is nowhere obstructed or interrupted, it is the Nir- manakaya.
These three being complete and fully present as one, are its very essence}
The three kayas, then, refer to these three intrinsic aspects of our enlightened mind; they also, of course, refer to different capacities of our perception. The vast majority of us are lim- ited in our vision, and only perceive the Nirmanakaya dimen- sion of form and manifestation. This is the reason that for most of us the moment of death is a blank and a state of oblivion, for we have neither encountered nor evolved any way of recognizing the Dharmakaya reality when it arises as the Ground Luminosity.
Nor do we have any hope of recog- nizing the Sambhogakaya fields as they appear in the bardo of dharmata. Because our entire life has been lived out within the realm of the impure perceptions of the Nirmanakaya manifes- tation, so at the moment of death we are transported directlyback into that dimension; we awaken, frantic and distracted, in the bardo of becoming in the mental body, taking illusory experiences for solid and real just as we have in lives before, and stumbling helplessly, propelled by past karma, toward rebirth.
Highly realized beings, however, have awakened in them- selves a perception completely different from our own, one that is purified, evolved, and refined to such an extent that, while they still dwell in a human body, they actually perceive reality in a totally purified form, transparent to them in all its limitless dimension. And for them, as we have seen, the expe- rience of death holds no fear or surprises; it is embraced, in fact, as an opportunity for final liberation.
-- The Tibetan Book of Living and Dying, p. 347-348
The earliest reference is from the Pali Tipitaka, AN 5.49-50:
49. “pabhassaramidaṃ, bhikkhave, cittaṃ. tañca kho āgantukehi upakkilesehi upakkiliṭṭhan”ti.
50. “pabhassaramidaṃ, bhikkhave, cittaṃ. tañca kho āgantukehi upakkilesehi vippamuttan”ti.
Luminous, o Bhikkhus, is this mind. And indeed by those invading defilements it is defiled.
Luminous, o Bhikkhus, is this mind. And indeed from those invading defilements it is released.
Buddha nature is luminosity of mind seeing perfection in every moment, every phenomena, every living beings i.e. not a single taint of imperfection. Needs a Path to bring Perfection into fruition - 84,000 of them. Choose one (or more) that suits.