When I am really drunk, I consciously experience hours of life which, upon awakening from a blackout several hours later, I have no recollection.
We know the brain stores memories, and we know the brain is necessary for conscious experience as we can relate it. It seems to me like our narrative concerning conscious experience is inextricably tainted by the reliance on memory, which is quite obviously an imperfect record of conscious experience and which has the unfortunate property of being housed in the same organ we propose creates our conscious experience. That is to say: we rely on our memories of conscious experience to say that we are conscious now, while we are alive, but that we were not prior to birth and after death. If I had no memories, and you asked me if I was conscious yesterday, we would not be able to say that we were. If no one told me that I took my pants off and danced on the table at Denny's last night after I got five pints in me, I might have no basis to believe that it happened.
The fundamental problem I'm getting at here is that conscious experience is a reported phenomenon, and one which relies on an unreliable source. This is such an intractable problem that some have taken to arguing consciousness is in fact an illusion, to try to escape the problem another way - but to me this seems only to ask "who/what is being deceived?" or "does it matter whether it is an "illusion" or real," since it is the experience which matters so much to us that we afford it very special legal status, regardless of whether it is "real."
The problem also sometimes is answered by investigating the neural correlate(s) of consciousness. If we can establish a neural correlate, then we can definitively say whether a brain is conscious or not. If I'm blackout drunk, you might plausibly inspect my brain and say "ah, but you are conscious right now; you just won't remember it later. I know this because you possess a brain state which corresponds to your normal waking experience." To many, however, this misses the point: the counterargument could go that you are not really escaping people's reported states: i.e. "yes, we all agree this is a brain state that correlates with what we call conscious experience." It does not provide evidence against the presence of conscious experience in systems which go unreported, because it does not establish that conscious states can only be realized in living brains, as opposed to other systems like slime molds, ant colonies, computers, or galactic dust clouds. Some think there may be special properties to certain systems which render them capable of conscious experience while others are not - this Type physicalism has its own thorny problems.
This leads some to escape the problem by ascribing conscious experience to all materials. There is something it is like, perhaps, to be an iron atom, so to speak - it's just that the iron atom has no way of remembering this or reporting it. Perhaps, they continue, conscious experience is a universal property of matter, and we are simply memory chauvinists. There are fun problems with this approach, too - if all things are conscious, then what does it mean that sometimes larger systems of matter, like human brains, apparently enjoy a conscious experience whose frame of reference is appropriately the system, not any of its individual constituents - though nothing is added to reality by the consideration of the system as whole? Maybe, they might answer, the proper frame of reference is the system as a whole - the universe. Any subsets of its contents may believe themselves individuals, but in fact this is like the conscious mind, blissfully unaware of its pervasive connection to the unconscious by dint of some quality in its present circumstance.
But by that point we are well beyond the realm of plausible investigation, and I am starting to think I should get a few more pints in me.
