A quick explanation of self-inquiry, itself just a way of seeing through a habitual way of thinking.
In How to Find What Isn't Lost, I mention various states of mind: sleep, dream, waking, what I call "stillness of mind," and what I call "the spacious mind."
I want to elaborate on some of this characterization of these states by putting them into a little structure based on certain notions from the mystical school of Advaita Vedanta.
What prevents us -- or seems to prevent us, to be precise -- from knowing the true Self is what is called ignorance.
Ignorance is basically of two varieties, of which only one is really problematic. They are both structural qualities of the mind. The first is the veiling quality (in Sanskrit, avarana), which is like a shadow, a darkness, an absence, or a forgetfulness, and which hides (or seems to hide) the light of the Self. It only seems to hide it, because the light by which we know the absence of the Self -- the light by which we see that incomplete perspective, that forgetting -- is itself nothing other than the light of the Self.
Then there is that aspect of the mind that makes it appear to be composed of many parts and pieces -- its variety and movement (in Sanskrit, vikshepa), its manifestation.
Now it's crucial to understand the relationship between these two pieces. They are interrelated and interdependent. It is from the veiling that all variety springs. Variety springs from the ego, the sense of a separate self, of a division between "I" and "other" -- this is the veiling of the Self. And it is what enables normal experience. You can't have any experiences without that division.
So that division, that veiling, is a certain mode of thought. And it sustains itself by clinging to various identities -- "I am an x..." -- be it father, mother, student, good person, bad person, great bowler, lover of the X-Files or whatever. So variety springs from veiling, but the veiling continues by clinging to the thoughts that are constantly generated in variety.
All the states of consciousness partake of these two qualities in different degrees.
That said, at the extremes, the two exist apart. That is, in deep sleep, the veiling exists in a potential, latent, seed-form -- a seed which actually means total veiling -- but without any manifestation. Conversely, in enlightenment, the variety exists but without the veiling, or rather -- the veiling is present, but in a harmless way.
In the normal dreaming and waking states, the veiling generates manifestation. What does this mean? It means that our normal sense is actually a series of senses of self -- every object, every new experience, is actually a two-faced thing (see the image at top, the two-faced Greek god Janus).
Now this manifestation, which comes from the veiling, is then a series of experiences which are half "sense-of-I" and half object, linked in one whole, unified experience-of-division. And it is has several problematic effects, which are really all different ways of saying the same thing.
It is a series of distractions, that is, variety, which prevents concentration on anything -- including and especially the spiritual search. Variety also, by distraction, hides the reflective quality of the mind, in the same way that ripples on a pond muddle the reflection of the sun. It is by that reflective, aesthetic quality of awareness that happiness manifests. So this mental variety and distraction a) blocks the spiritual search, which leads to the destruction of the veil and b) blocks the experience of happiness.
Reduce the variety and distraction and you lead to solutions on both goals. So therefore, one is advised to obtain a quiet mind, which means a mind in which the variety is reduced to a minimum, without being totally absent. Variety has nothing to do with the number of sense perceptions and everything to do with how they're reacted to in the mind.
In contrast to this, there is what I call the "spacious mind." This is a state of still mind which is still informed by some kind of intention. It is consciously reached by the processes of self-inquiry and surrender I define in How to Find What Isn't Lost. It also manifests as the state of "flow" sometimes when you are engaged deeply in an activity. In it, you are conscious and can work, but still have that stillness. That is, the mind is still but it does not go to sleep. The spacious mind is effortless, conscious, and high-performance. In the spacious mind, the veiling is removed, but variety remains. Usually, one cannot stay in the spacious mind because of various habits that pull you out of it, that pull you back to your ordinary self. Enlightenment means those habits have been conquered and the spacious mind is recognized as your natural and permanent state.
As you see from above, being zoned out and being in a state of mental stillness are very close, the key difference being that being zoned out is reached accidentally, and so is less conscious and more veiled. Even stillness, however, can become mere sleep if there is no further goad to action.
I'll also note that even "in" enlightenment, so to say, the quietness of mind -- that is, the lack of manifestation -- still matters for the experience of happiness. Enlightenment is the knowledge that the Self is pure happiness. But if the mind wishes to experience it, it must be quiet. Quietness means few thoughts, little distraction... so that the blissfulness of the Self shines through. Few thoughts does not mean dullness or inaction; it means that what is thought about is thought about in a focused manner.