In the current issue of the Journal of Consciousness Studies, Keith Frankish argues that consciousness is an illusion -- or at least that "phenomenal consciousness" is an illusion. It doesn't exist.
Now I think there are basically two different things that one could mean in saying "consciousness doesn't exist".
(A.) One is something that seems to be patently absurd and decisively refuted by every moment of lived experience: that there is no such thing as lived experience. If it sounds preposterous to deny that anyone ever has conscious experience, then you're probably understanding the claim correctly. It is a radically strange claim. Of course philosophers do sometimes defend radically strange, preposterous-sounding positions. Among them, this would be a doozy.
(B.) Alternatively, you might think that when a philosopher says that consciousness exists (or "phenomenal consciousness" or "lived, subjective experience" or whatever) she's usually not just saying the almost undeniably obvious thing. You might think that she's probably also regarding certain disputable properties as definitionally essential to consciousness. You might hear her as saying not only that there is lived experience in the almost undeniable sense but also that the target phenomenon is irreducible to the merely physical, or is infallibly knowable through introspection, or is constantly accompanied by a self-representational element, or something like that. Someone who hears the claim that "consciousness exists" in this stronger, more commissive sense might then deny that consciousness does exist, if they think that nothing exists that has those disputable properties. This might be an unintuitive claim, if it's intuitively plausible that consciousness does have those properties. But it's not a jaw dropper.
Admittedly, there has been some unclarity in how philosophers define "consciousness". It's not entirely clear on the face of it what Frankish means to deny the existence of in the article linked above. Is he going for the totally absurd sounding claim, or only the more moderate claim? (Or maybe something somehow in between or slightly to the side of either of these?)
In my view, the best and most helpful definitions of "consciousness" are the less commissive ones. The usual approach is to point to some examples of conscious experiences, while also mentioning some synonyms or evocative phrases. Examples include sensory experiences, dreams, vivid surges of emotion, and sentences spoken silently to oneself. Near synonyms or evocative phrases include "subjective quality", "stream of experience", "that in virtue of which it's like something to be a person". While you might quibble about any particular example or phrase, it is in this sense of "consciousness" that it seems to be undeniable or absurd to deny that consciousness exists. It is in this sense that the existence of consciousness is, as David Chalmers says, a "datum" that philosophers and psychologists need to accept.
Still, we might be dissatisfied with evocative phrases and pointing to examples. For one thing, such a definition doesn't seem very rigorous, compared to an analytic definition. For another thing, you can't do very much a priori with such a thin definition, if you want to build an argument from the existence of consciousness to some bold philosophical conclusion (like the incompleteness of physical science or the existence of an immaterial soul). So philosophers are understandably tempted to add more to the definition -- whatever further claims about consciousness seem plausible to them. But then, of course, they risk adding too much and losing the undeniability of the claim that consciousness exists.
When I read Frankish's article in preprint, I wasn't sure how radical a claim he meant to defend, in denying the existence of phenomenal consciousness. Was he going for the seemingly absurd claim? Or only for the possibly-unintuitive-but-much-less-radical claim?
So I wrote a commentary in which I tried to define "phenomenal consciousness" as innocently as possible, simply by appealing to what I hoped would be uncontroversial examples of it, while explicitly disavowing any definitional commitment to immateriality, introspective infallibility, irreducibility, etc. (final MS version). Did Frankish mean to deny the existence of phenomenal consciousness in that sense?
In one important respect, I should say, definition by example is necessarily substantive or commissive: Definition by example cannot succeed if the examples are a mere hodgepodge without any important commonalities. Even if there isn't a single unifying essence among the examples, there must at least be some sort of "family resemblance" that ordinary people can latch on to, more or less.
For instance, the following would fail as an attempted definition: By "blickets" I mean things like: this cup on my desk, my right shoe, the Eiffel tower, Mickey Mouse, and other things like those; but not this stapler on my desk, my left shoe, the Taj Mahal, Donald Duck, or other things like those. What property could the first group possibly possess, that the second group lacks, which ordinary people could latch onto by means of contemplating these examples? None, presumably (even if a clever philosopher or AI could find some such property). Defining "consciousness" by example requires there to be some shared property or family resemblance among the examples, which is not present in things we normally regard as "nonconscious" (early visual processing, memories stored but not presently considered, and growth hormone release). The putative examples cannot be a mere hodge-podge.
Definition by example can be silent about what descriptive features all these conscious experiences share, just as a definition by example of "furniture" or "games" might be silent about what ties those concepts together. Maybe all conscious experiences are in principle introspectively reportable, or nonphysical, or instantiated by 40 hertz neuronal oscillations. Grant first that consciousness exists. Argue about these other things later.
In his reply to my commentary, Frankish accepts the existence of "phenomenal consciousness" as I have defined it -- which is really (I think) more or less how it is already defined and ought to be defined in the recent Anglophone "phenomenal realist" tradition. (The "phenomenal" in "phenomenal consciousness", I think, serves as a usually unnecessary disambiguator, to prevent interpreting "consciousness" as some other less obvious but related thing like explicit self-consciousness or functional accessibility to cognition.) If so, then Frankish is saying something less radical than it might at first seem when he rejects the existence of "phenomenal consciousness".
So is consciousness an illusion? No, not if you define "consciousness" as you ought to.
Maybe my dispute with Frankish is mainly terminological. But it's a pretty important piece of terminology!