These two features - luminosity, or clarity, and knowing, or cognizance - have come to characterise 'the mental' in Indo-Tibetan Buddhist thought. Clarity here refers to the ability of mental states to reveal or reflect. Knowing, by contrast, refers to mental states' faculty to perceive or apprehend what appears.
All phenomena possessed of these qualities count as mental. These features are difficult to conceptualise, but then we are dealing with phenomena that are subjective and internal rather than material objects that may be measured in spatio-temporal terms.
Perhaps it is because of these difficulties - the limits of language in dealing with the subjective - that many of the early Buddhist texts explain the nature of consciousness in terms of metaphors such as light or a flowing river. As the primary feature of light is to illuminate, so consciousness is said to illuminate its objects. Just as in light there is no categorical distinction between the illumination and that which illuminates, so in consciousness there is no real difference between the process of knowing or cognition and that which knows or cognizes. In consciousness, as in light, there is a quality of illumination.
~ His Holiness the Dalai Lama, "The Universe in a Single Atom"
"...In training ourselves to take consciousness itself as the object of first-person investigation, we must first stabilise the mind. The experience of attending to the mere present is a very helpful practice. The focus of this practice is a sustained training to cultivate the ability to hold the mind undistractedly on the immediate, subjective experience of consciousness. This is done as follows.
Before one begins formal sitting meditation, one develops a deliberate intention not to allow the mind to be distracted either by recollections of past experience or by hopes, anticipations and fears about future events. This is done by making a silent pledge that during this meditation session the mind will not be seduced by thoughts of the past or the future and that it will remain fully focused on awareness of the present. This is critical because in our everyday normal states we tend to be tied either to recollections and vestiges of the past or to hopes and fears about the future. We tend to live either in the past or in the future and very rarely fully in the present. When one is actually in the meditation session, it may be helpful to face a wall that has no contrasting colours or patterns that might distract the attention. A muted colour like cream or beige can be suitable, for these help create a simple background. When one is actually in the session, it is critical not to apply any exertion. Rather, one must simply observe the mind resting naturally in its own state.
As one sits, one will begin to notice that all sorts of thoughts arise in the mind, like a bubbling spring of never-ending internal chatter or the bustle of endless traffic. One should allow whatever thoughts arise to do so freely, regardless of whether one perceives them as wholesome or unwholesome. Do not reinforce them or repress them or subject them to evaluative judgement. Any of these responses will create further proliferation of thought, for it will provide the fuel that keeps the chain reaction going. One must simply observe the thoughts. When one does this, just as bubbles arise and dissolve into water, the discursive thought processes simply arise and dissolve within the mind.
Gradually, in the midst of the internal chatter, one will begin to glimpse what feels like a mere absence, a state of mind with no specific, determinable content. At the beginning, such states may be only fleeting experiences. Nevertheless, as one becomes more experienced in this practice, one will be able to prolong the intervals in one's normal proliferation of thought. Once this happens, there is a real opportunity to understand experientially what is described in the Buddhist definition of consciousness as 'luminous and knowing'. In this way a meditator will gradually be able to 'grasp' the basic experience of consciousness and take that as the object of meditative investigation…”
~ His Holiness the Dalai Lama