When thinking of hallucination I'm drawn to a set of questions which we might summarise with 'What is the being of hallucination?' The question itself naturally invites another question, which is: 'What is it to ask into the being of a thing?' And: how does this differ from an empirical-psychological inquiry?
An empirical theory of hallucination might have it that hallucinations are caused by stress, have a symbolic content related to the hallucinator's complexes, involve abnormal activity in the superior temporal gyrus, etc. Such theories are all well and good, but here I note them only to provide a contrast to the kind of enquiry I'm instead inclined to pursue. They are answers to a different question (to a question individuated differently at the level of sense if not at the level of surface expression). They tell us, one might say, what happens when you hallucinate, but not what it is to hallucinate.
An empiricist might show impatience with the question of the being of hallucination, and propose a definition, after which we can (it is suggested) move swiftly on to the more important empirical matters at hand. You know the kind of thing: 'A hallucination is a perception / sense datum / inner experience / inner representation - in the absence of / not caused by - an outer object'.
Yet our question has its place precisely because of the futility of those kinds of answers, because of how wedded they are to a bankrupt inner/outer picture which reifies the inner and correlatively constitutively divorces experience as such from our world-involvement. For perception and experience are not just caused by their worldly objects; they take them in. They are not hybrids of i) non-mental causal outer interactions with a world (the mechanics of vision and audition etc) plus ii) mental upshots of 'loud and glowing sense data' in an internal world. Instead perceptual experience is our openness to the world; it is the originary form of intentionality. In this sense a hallucination is precisely not a perception or an experience of anything. For there is, one could say, something simply paradoxical in talking of 'an experience of a horse in the absence of a horse'.
Furthermore the concepts of 'inner representation', 'sense datum' etc are simply philosophical inventions which themselves cry out for explication before they themselves can feature in explanations. After all, one could say, what use is a visual representation if we can't actually see it? Doesn't the concept of a 'visual representation' - e.g. of a picture - presuppose rather than explain the capacity to see what is thereby pictured? (Consider our ability to see the pictures hanging on the wall, and how derivative this ability is of our ability to first see, directly, the kinds of things the pictures picture.) So we can't just help ourselves to such notions to explain perception. We need first to explain them. It won't do to say that an inner representation is self-perceiving or self-interpreting, since to say such things is far less perspicuous than talk of perceptual experience itself.
Consider too the arguments for disjunctivism in the philosophy of perception. The non-disjunctivist, to rehearse, has it that the visual perception of a horse, and a hallucination of a horse, have something psychological in common. The term 'psychological' there is doing the work of: they don't just have in common the atemporal fact that the right way to describe their content is 'a horse'. It is saying: they have in common something experiential and episodic. They are not just both experiences of horses; they are both - some or other allegedly illuminating sense - experiences of horses. Yet the disjunctivist will demur that it is no more illuminating to say this than to say that a real horse and a plastic horse are both horses, or that a standing bridge and a bombed out bridge are both bridges. We can of course say that a hallucination is a perceptual experience, just as we can also say that a bombed out bridge is a bridge. Yet in both cases, the disjunctivist insists, what is essential to the being of the perceptual experience (openness to the world) and the bridge (forging a connection between two sides of a river) has been lost. The reason why we identify the broken bridge as a bridge, the plastic horse as a horse, the hallucination as an experience, has to do with their ontological dependence on real bridges, horses, perceptions.
A hallucination, on this natural understanding, is a particular kind of disturbance in a perceptual modality. There is no more an experience left to it than there is a bridge left in the case of the bombed out bridge, or a horse in the plastic horse. We can call both veridical perceptions and hallucinations 'experiences', but this is not because they share something episodic in common, but instead because reference to all such phenomena gives us the extension of the concept of 'experience'. It wouldn't be elucidatory to say, for example, that in both real and hallucinatory cases it seems to us that there is a horse in front of us. For first of all it may not seem to the hallucinator that there is a horse in front of her. It may instead seem to her just that she is hallucinating a horse. (Merleau-Ponty is very clear on this.) And second it isn't clear what it means to say of someone who clearly sees a horse that it seems to him that he sees a horse. And this is because part of the work that the concept of 'seeming' does is to distinguish between, for example, when something 'really is' the case and when something 'just' seems to be the case. To say that there is a 'seeming' alive in both cases sublimes the logic of 'seems'.
It is for reasons such as these that I consider it important to ask 'what is being of hallucination?' In spelling this out it will naturally be fine to say things like 'It's kind of like seeing something but that something isn't actually seen'. In such cases we often talk about 'hearing things' and 'seeing things'; more specifically we talk of 'hearing voices' and 'seeing visions'. In doing this we indicate that other sensory modalities are not involved. That is the force of talk of 'hearing voices'. After all it could be said that in one sense we all hear voices everyday; or, at least, we listen to what people are saying. But the talk of hearing voices indicates that the talker is not visually or otherwise present to us. It is in the spirit of this question that I propose my answer: to hallucinate is to have an embodied expectation of hearing (or seeing, being touched, etc.) uncancelled by the absence of a stimulus. I do not offer that as an empirical theory but as a phenomenological characterisation. I do not imagine for a moment that such talk of 'embodied and cancelled expectations' is straightforwardly perspicuous. Instead I illustrate it with examples (such as the static escalator appearing to lurch when we embark it, or the world appearing to still spin around after you've stopped spinning around).
What I am not doing is providing necessary and/or sufficient conditions for 'hallucinate'. Such an analytic approach would, I believe, radically underestimate the fundamental nature of hallucination as a disturbance of experiential world engagement. As for perception itself, to take the question of the being of perception as a request for necessary and sufficient conditions for perception is to tacitly imagine that there are floating about some more fundamental concepts which we can independently grasp and then put to use in our characterisation of what it is to perceive. That, I submit, is self-evidently absurd. If one says that 'perception is our originary openness to the world', that it 'takes us out to the objects', that it involves an 'originary transcendence', I hope it is clear how these can hardly be taken as statements conveying positive information. Instead they are reminders not to make a travesty of our concept of perception by closing us in to an inner mental domain in a merely external relation with the world about us.
No, we mustn't here try to achieve an illuminating definition, but instead accommodate to, find our way about with, the concept of 'perception' in practice. The same goes for 'hallucination'.