It would be best to begin with the consciousness-aggregate (viññāṇa). Consciousness deserves first place because any experience means being conscious of the other four aggregates. I am conscious of matter (my body, an external object, or both), I am conscious of feeling, I am conscious of perception, and I am conscious of determination.
However, what is meant by being conscious of something? When I say “I am conscious” of something, it means that the thing of which I am conscious is present in me. A sight, a sound, a smell, a taste, a touch or a mental image is present. A pleasant feeling, an unpleasant feeling, or a neither-pleasant-nor-unpleasant feeling is present. It is present to me; therefore, I am conscious of it.
It is quite clear, in experience, that consciousness without an object of which one is conscious—‘pure consciousness’—is impossible and inconceivable. Further, the object of consciousness itself cannot be consciousness. That is, we cannot have ‘consciousness of consciousness’—which would be equivalent to ‘presence of presence’. We can have consciousness only of matter, feeling, perception and determination. Also, we cannot think or speak of any object that has no relation with consciousness. Thought we speak of consciousness and objects separately, they are inseparable in experience; speaking of them separately is mere verbal abstraction. When we speak in an abstract way of ‘consciousness’, the Suttas tell us we are making an ‘aggregate-designation’ (khanda-adivacana) But when we speak of ‘consciousness of X’, we are not speaking in the abstract but of a concrete and particular thing.
As we already pointed out, consciousness (viññāṇa) should not be mistaken for awareness (sampajañña). ‘Consciousness of movement’ and ‘awareness of consciousness of movement’ are not the same. Awareness involves consideration to some degree. What is often called ‘unconscious’ should really be called ‘unaware’. When someone is asleep, we cannot say that he is unconscious. Were it so, he could not be woken out of sleep by a noise or other disturbance. But we can say that he is not aware.
The necessity to differentiate between consciousness and first-order awareness has given rise to terms like ‘unconscious’, ‘subconscious’, ‘semiconscious’, etc. These words essentially refer to the degree of consciousness involved in immediate experience (conscious of X), but not in awareness of the experience (conscious of X as X). The Suttas do not speak of different grades of consciousness such as ‘unconscious’, ‘subconscious’, etc.; however, they do speak of a state where feeling and perception have ceased, and therefore consciousness has also ceased, but but there is heat and vitality in the body, and so there is a return to consciousness at the time predetermined before the attainment of the state. This is called the attainment of the cessation of perception and feeling (saññavedayitanirodhasamāpatti).
Now, as regards essence, consciousness is negative; it cannot be seen, heard, smelled, tasted or felt. Also consciousness does not refer to the phenomenon that is present, or even to part of the phenomenon. It is not what is present, or even a part of what is present. It is only the presence of the phenomenon. Consciousness is the presence of that which is present. In an immediate visual experience, the thing is seen but eye-consciousness is not seen. Eye-consciousness is negative as regards essence. But in reflexive visual experience, eye-consciousness is present—there is eye-consciousness.
Thus, while other things can be directly described in terms of their positive essence as ‘this thing’ or ‘that thing’, consciousness cannot be so described. It can be described only upon reflexion. Yet, if anything is to be described, it must be present, at least in imagination; and its presence is consciousness. So we have a situation where other things depend upon consciousness for their existence (whether in this form or that form), and that merely means that consciousness is the existential determination.
Since consciousness is negative regarding essence, it is always associated with the body; it is, in the words of the Buddha:
Ettha sita ettha paṭibaddha: “fastened there, bound there.” — Mahāsakuludāyi Sutta (Majjhima-Nikāya 77)
Therefore in the Suttas we find the phrase:
Saviññāṇaka kāya: “body endowed with consciousness.” — Rādha Sutta (Saṃyutta-Nikāya 22.71)
Normal experience, however, is multiple; there are hearing, seeing etc. Experience is not confined to one sense or faculty, so we can distinguish six kinds of consciousness, arising dependent on six bases (saḷāyatana): eye, ear, nose, tongue, body and mind.
Monks, whatever consciousness arises dependent upon, by that it is reckoned. Consciousness arising dependent on the eye and sight is reckoned as eye-consciousness; consciousness arising dependent upon the ear and sound is reckoned as ear-consciousness; consciousness arising dependent upon the nose and odor is reckoned as nose-consciousness; consciousness arising dependent upon the tongue and taste is reckoned as tongue-consciousness; consciousness arising dependent upon the body and touch is reckoned as body-consciousness; consciousness arising dependent upon the mind and mental images is reckoned as mind-consciousness.” — Mahātaṇhāsaṅkhaya Sutta (Majjhima-Nikāya 38)
Each of these six sense bases is described as that thing which is:
Lokasmiṃ lokasaññī hoti lokamāni: “in the world and by which there is world-perceiving and world-conceiving.” — Lokantagamana Sutta (Saṃyutta-Nikāya 35.99)
Thus the eye can be described as that spherical organ in the world by which there is seeing (sight-perception) of the world and conceiving of the world; the ear can be described as a membranous organ by which there is hearing (sound-perception) of the world and conceiving of the world; and the nose, tongue, and body can be similarly described. And the mind (mano) can be described as that organ—principally the gray matter of the brain—by which there is imagining (idea-perception or mental-image-perception) of the world and conceiving of the world. Detailed physiological descriptions are unnecessary; the functional descriptions given in the Suttas are quite sufficient.
But the description of the mind-base appears insufficient and inadequate, because although there can be no hearing based on the eye-base or seeing based on the ear-base, etc., there can be a sort of seeing, hearing, smelling, tasting and touching based on the mind-base. Based on the mind-base there can be imaginary sights, sounds, smells, tastes and touches. From this point of view, the mind-base can be thought of as incorporating five imaginary bases, in which five kinds of imaginary perceptions can arise. However, some exclusively mental phenomena such as telepathy, clairvoyance, retrocognition, precognition, etc. may not conform entirely to this scheme.
Thus considering all the six sense bases, we have consciousness defined in the Suttas as follows:
Monks, what is consciousness (viññāṇa)? It is these six consciousness-bodies (or consciousness-groups): eye-consciousness, ear-consciousness, nose-consciousness, tongue-consciousness, body-consciousness and mind-consciousness. This, monks, is called consciousness.” — Upādānaparipavatta Sutta (Saṃyutta-Nikāya 22.56)
The earlier phrase “body endowed with consciousness” can now be expanded to “six-based body endowed with consciousness” (saviññāñaka saḷāyatanika kāya)