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: 
I can't deny myself (the 'I' or the consciousness of being myself). Can you? If you say "Yes", you would never have asked your question. In other words, this question must be asked unconsciously. If you say "Yes" again, why can't you say the consciousness for asking this question is somewhere else? So....

**One can deny everything except consciousness (though actually it is Pure consciousness).**

But there is a chance for denial of consciousness until that person meets a great Guru. In other words, if he says so, that means he has never met such a person. 

In short, I would say, if you try to weld the terms 'consciousness' and 'illusion' the latter burns completely without leaving out a sign of its 'particles'. Our sense organs will become useless after our death, so it cannot show you the 'real knowledge'. The real knowledge must be beyond all the knowledge acquired through our sense organs.

> 
> 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?
> 
> 
> 

In enlightenment, when the duality disappears, the observer, the observed and the act of observing all become one. Consciousness is not an illusion.

> 
> Is there any coherent argument in support of consciousness being an
>  illusion?
> 
> 
> 

If there is one such argument, it must never be coherent.

<https://en.wikipedia.org/wiki/Consciousness>

<https://en.wikipedia.org/wiki/Chaitanya_(consciousness)>
Your scepticism of Dennett is well-grounded. Claims similar to Dennetts were given by Skinner under the rubric of behavioral psychology where one supposed thinking things (*res cogitans*) were merely things, that is automatons, and one was not supposed to ask a man about his inner self, his *telos* or his intentions, or indeed how things were going with him, as one was supposed to merely observe his actions, his gestures, his attitudes and then tell him truly how things were going with him, his intentions, his purposes. 

These claims were blown apart by Chomsky who pointed out the chicanery of supposing human beings were to be studied as though they were merely objects of the disinterested scientific gaze - like rocks or laboratory rats. The chicanery goes deeper as it also sullies the good name of science by a methodology that simply doesn't stand up to any scrutiny. Likewise Dennett is sullying the good name of philosophy by dogmatically flogging a dead thought to a coterie of admirers who should know better - but don't - and to a larger audience who do know better but are cowed or over-awed by his linguistic gymnastics. 

Every language under the sun has the word 'I'. Self-awareness, self-consciousness exists. It's not an illusion. The only illusion here is that Dennett is doing good philosophy when he's doing bad philosophy.