I'll start by acknowledging that this isn’t my idea, credit to Sam Harris. I also don’t know if this is even controversial, but I figured this would be a better place to post than in Showerthoughts.

By consciousness, I mean the subjective experience of what it feels like to be. As philosopher Thomas Nagel put it:

'An organism has conscious mental states if and only if there is something that it is like to be that organism—something it is like for the organism.'

It’s at least conceivable that things like free will, the self, or even the entire universe could be an illusion. For all we know, we could be living in a simulation and nothing might be real. Even if you don’t believe that, there’s still a greater-than-zero chance you could be wrong. However, this doesn't apply to consciousness itself. Even if everything is just a hallucination, it remains an undeniable fact that it feels like something to hallucinate. To claim that consciousness could be an illusion is a self-contradictory statement as consciousness is where illusions appear.