Question: Whenever I hear someone like, say, Daniel Dennett say that consciousness is an "illusion", I'm always left pretty much stumped. Firstly, both the terms "consciousness" and "illusion" are generally left undefined, although this is more apparent with the term "illusion". With this, I have to use the default definitions of these terms, "consciousness" being a somewhat self-explanatory "state of conscious awareness", and "illusion" being "a delusion of some sort". This is really where the main problem arises for me. How can one claim that consciousness is an illusion when the concept of "illusion" requires a subject to be deluded, which the original claim actively denies the existence of? Granted, this subject doesn't have to be an "I", but the denial of any decieved subject at all seems to negate the argument itself.

The other problem I have with the claim is that it seems to contradict the fairly self evident claim that, as Nietzsche put it, "It thinks, therefore it is".

Is the claim really as nonsensical as it seems, or am I missing something big? Perhaps the definition of a term or multiple terms?

Answer: 
This phrasing is used by different philosophers for different purposes, so one must take the phrase in context. However, in general, it is used in reference to the idea that consciousness is a thing which is separable from other things.

Denett argues that consciousness does not exist except as an image which describes the interplay of events within the brain. I use the word image here to avoid the loaded term "illusion." The key factor here is that the image is a depiction of the thing rather than the thing itself. Denett would argue that if one were to take this image and look for it in the real world, we would not find it. Instead we would find the interplay of events in the brain. In this sense, I might argue that his position is that consciousness is to reality as the number 1 is to the series {1/2, 3/4, 7/8, 15/16, ...}. It could be viewed as a limit were the process to be continued, but it never actually reaches that point. In his view, the reality never quite reaches the vaunted concept we call consciousness.

In this sense, the word illusion is chosen for pragmatic reasons. It instills the correct imagery in the minds of those who believe consciousness exists. For those who agree with Denett, the word must naturally shift meaning, for there is no consciousness to ascribe meaning to the word "illusion." There may be a self-consistent viewpoint regarding how the brain *really* works such that that phrase is interpreted correctly, but Denett may not call this out with extra words.

Other philosophers have used this concept for other reasons. Alan Watts, for instance, argues that no self is truly isolated from its environment. According to him, there is no consciousness which can be isolated from its environment and pointed at to say "this is a consciousness." He argues that "for every inside there is an outside." Thus there is no consciousness outside or isolated from its environment.
When I run across problematic statements of this sort, I always prefer to follow Wittgenstein and look for the error in language that lies behind it. This is precisely the kind of 'philosophy' he thought needed therapy more than analysis.

The first step in that Wittgensteinian therapy is to step back and consider what *purpose* this claim has in Dennett's worldview. I don't mean to ask what the claim *means*, per se. Instead, I want to think of this claim as a move in, say, an abstract, philosophical game of chess, and ask what line of progression this claim is meant to block, or what intellectual territory it is attempting to claim. Looked at that way, the purpose is almost self-evident: the concept of 'consciousness' — ill-defined as it is — has long been associated with metaphysical and religious concepts like God and the soul and moral behavior. Disposing of consciousness immediately undercuts a wide range of philosophical arguments from theology, moral philosophy, and social theory. Since Dennett's *raison d'être* has always been anti-Abrahamic anti-theism, this kind of claim makes perfect practical sense. Whether it makes *philosophical* sense is almost irrelevant, because the position is too useful to the ultimate goal of the intellectual contest to be given up without a fight. This is why he settles on the idea that consciousness is an 'illusion': he cannot deny that we experience something we call consciousness, so instead he tries to pry it away from any direct ontological referent and reconstruct it as an emergent phenomenon of interconnected organic systems. 

Of course, this is just scientism: speculative philosophy based on an over-extension of scientific observations. 'Consciousness is an illusion' is no more empirically verifiable than 'consciousness is the product of a divine soul'. But even as mere scientism it creates enough ambiguity to undercut religious certainty on the matter, so it serves that purpose well.

The therapy in this case is to recognize that neither Dennett nor his opponents have any idea what consciousness *is*, but all acknowledge that the term points at *something*. The rest is merely wrangling over lexical issues: efforts to assert one formulation or another as *a priori* true. We can (if we choose) skip all that as contested territory, and move forward to actual substance of the game, which is over the sociopolitical balance of power between religious and secular segments of modern society.