In terms of the aggregate of consciousness (vinnana-khandha), it's clear that consciousness is a dependently existing phenomena. Sensory consciousness can only arise with the presence of the appropriate sense organ and its corresponding object of reference. The process of seeing, for example, is described as a conditional process where "dependent on eye and visible forms, eye-consciousness arises" (SN 12.43). Without the presence of the appropriate sense organ (e.g., the eye) or the corresponding object of reference (e.g., rock), sensory-consciousness (e.g., eye-consciousness) can't arise. So none of the six forms of sensory consciousness can stand on its own without the corresponding stimulus to make it manifest or arise.
Nevertheless, there are a couple of sutta passages that seem to suggest there's a form of consciousness that doesn't come under the aggregate of consciousness. For example, Thanissaro Bhikkhu states in a note to his translation of MN 109:
One form of consciousness apparently does not come under the aggregate of consciousness. This type of consciousness is termed vinnanam anidassanam — consciousness without a surface, or consciousness without feature. MN 49 says specifically that this consciousness does not partake of the "allness of the all," the "all" being conterminous with the five aggregates. The standard definition of the aggregate of consciousness states that this aggregate includes all consciousness, "past, present, or future... near or far." However, because vinnanam anidassanam stands outside of space and time it would not be covered by these terms. Similarly, where SN 22.97 says that no consciousness is eternal, "eternal" is a concept that applies only within the dimension of time, and thus would not apply to this form of consciousness.
There are those in academia who also acknowledge this possibility. Peter Harvey, professor of Buddhist Studies at the University of Sunderland, writes in his Introduction to Buddhism:
Nevertheless, certain passages in the Suttas hint that Nibbana may be a radically transformed state of consciousness (vinnana):
The consciousness in which nothing can be made manifest (like space), endless, accessible from all sides (or: wholly radiant):
Here it is that solidity, cohesion, heat and motion have no footing,
Here long and short, coarse and fine, foul and lovely (have no footing),
Here it is that mind (nama) and body (rupa) stop without remainder:
By the stopping of consciousness, (all) this stops here. (D.I.223)
Like Ud.80, above, this describes a state beyond the four physical elements, where mind-and-body are transcended. As the heart of Conditioned Arising is the mutual conditioning of consciousness and mind-and-body, this state is where this interaction ceases: from the stopping of consciousness, mind-and-body stops. Consciousness is not non-existent when it stops, however; for it is said to be non-manifestive and endless. One passage on the stopping (nirodha) of the nidana of consciousness (S.III.54-5) says that there is no longer any object (arammana) or support (patittha) for consciousness; consciousness is thus 'unsupported' (apatitthita) and free of constructing activities, so that it is released, steadfast, content, undisturbed, and attains Nibbana. This description, of a 'stopped' consciousness which is unsupported by any mental object, where mind-and-body are transcended, seems to accord well with the Ud.80 description of Nibbana itself.
To say that Nibbana is unconditioned, objectless consciousness indicates something of its nature, but it does not penetrate far into its mystery. For it seems impossible to imagine what awareness devoid of any object would be like. As regards the 'stopping' of mind-and-body, as a state occurring during life, this is perhaps to be understood as one where all mental processes (including ordinary consciousness) temporarily cease, and the matter of the body is seen as so ephemeral as not to signify a 'body'. A passage at M. I.329-30 which parallels D.I.223 says that the non-manifestive consciousness 'is not reached by the solidness of solidity, by the cohesiveness of cohesion...'. The analysis of Nibbana as objectless consciousness, though, is the author's own interpretation. Theravadin tradition sees Nibbana as 'objectless' (Dhs.I408), but regards 'consciousness' as always having an object. D.I.223 is thus interpreted as concerning Nibbana as to-be-known-by-consciousness: Nibbana is itself the object of the Arahat's consciousness (Pati.II.I43-5).
While the view that there's a type of consciousness that lies outside of space and time, and therefore, outside the consciousness-aggregate altogether, isn't a view that's supported by the 'classical' Theravada Tradition in which the entire Tipitaka and its commentaries are considered authoritative, the imagery of consciousness that "does not land or increase" mentioned in SN 12.64 does seem to support such a possibility, even if some might say that comparing this imagery of consciousness to the consciousness of nibbana is taking it out of context. At least I think so.
The commentaries, on the other hand, gloss the term vinnanam anidassanam in a way that denies such a possibility. Using the Kevatta Sutta (DN 11), for example, Suan Lu Zaw, a Burmese lay-teacher of Pali and Abhidhamma, explains that according the the Kevatta Sutta Atthakatha [DN 11 commentary], vinnanam doesn't refer to the usual meaning of 'consciousness' here, but instead defines it as, "There, to be known specifically, so (it is) 'vinnanam'. This is the name of Nibbana." He also explains that the following line of DN 11, "Here (in Nibbana), nama as well as rupa cease without remainder. By ceasing of consciousness, nama as well as rupa ceases here" illustrates this point. He states that, "Nibbana does not become a sort of consciousness just because one of the Pali names happens to be vinnanam."
He concludes by using a quote from a section of the Dhammapada Attakatha [Dhammapada commentary], which apparently states that there's no consciousness component in parinibbana after the death of an arahant. This, of course, is in direct contrast to Thanissaro Bhikkhu's note to this particular sutta.
Basically, what this controversy seems to boil down to is the experience of nibbana and the nature of that experience, especially after death. The general tendency is to either describe nibbana as the ending of all consciousness, all awareness, or in other words, to stress the cessation aspect of nibbana, or to describe nibbana as a state of purified awareness, 'consciousness without feature,' or in other words, to stress the transcendent aspect of nibbana. The 'classical' Theravada Tradition favours the former view of nibbana while others, especially some within the Thai Forest Tradition, favour the latter.
As I mentioned in another post, I used to lean towards the classical position that all consciousness ceases at death, and then I tended to lean more towards the view that there may be a type of consciousness that lies outside of space and time. The imagery of consciousness that "does not land or increase" mentioned in SN 12.64 does seem to support such a possibility, as do various other passages throughout the Canon. And while many assume nibbana to be equivalent to a state of nonexistence, a state of nothingness, Bhikkhu Bodhi points out in In the Buddha's Words: An Anthology of Discourses from the Pali Canon that, "… no text in the Nikayas ever states this. To the contrary, the Nikayas consistently refer to Nibbana by terms that refer to actualities. It is an element (dhatu), a base (ayatana), a reality (dhamma), a state (pada), and so on" (319).
Lately, however, I've been content with viewing nibbana as the extinguishing of greed, hatred and delusion (SN 38.1), and consciousness without feature as the living arahant's consciousness devoid of defilements and non-attached to any phenomena whatsoever, and leaving it at that. In fact, I think Bhikkhu Nanananda sums up this perspective well in "Nibbana Sermon 07":
Now viññāṇaṃ anidassanaṃ is a reference to the nature of the released consciousness of an arahant. It does not reflect anything. To be more precise, it does not reflect a nāma-rūpa, or name-and-form. An ordinary individual sees a nāma-rūpa, when he reflects, which he calls 'I' and 'mine'. It is like the reflection of that dog, which sees its own delusive reflection in the water. A non-arahant, upon reflection, sees name-and-form, which however he mistakes to be his self. With the notion of 'I' and 'mine' he falls into delusion with regard to it. But the arahant's consciousness is an unestablished consciousness.
We have already mentioned in previous sermons about the established consciousness and the unestablished consciousness.[ix] A non-arahant's consciousness is established on name-and-form. The unestablished consciousness is that which is free from name-and-form and is unestablished on name-and-form. The established consciousness, upon reflection, reflects name-and-form, on which it is established, whereas the unestablished consciousness does not find a name-and-form as a reality. The arahant has no attachments or entanglements in regard to name-and-form. In short, it is a sort of penetration of name-and-form, without getting entangled in it. This is how we have to unravel the meaning of the expression anidassana viññāṇa.