Consciousness, it is the one thing that even Descartes could bring no doubt to. We have our internal state, and this is a certainty, for to say that you don't have consciousness requires a consciousness with which to conclude this and as such the very proposition itself is self-defeating.
But, what is it?
The brain is undoubtedly the seat of consciousness. Just pick up a book by Oliver Sacks or watch a lecture by Ramachandran and you will see why. Not only are we able to find the correlations between everyone's centers for processing through things like MRIs, but we are also able to explore the world of consciousness through the anomalies within neurological disorders and how they affect people's consciousness.
For example: in Kapkra's syndrome we are able to see someone who has suffered a lesion between the Lateral Geniculate Body and the Amygdala and it can manifest in the person, otherwise totally cogent and themselves, will see their mother and conclude that though it looks and sounds like his mother, it is an imposter. This is because the pathways which are responsible for attaching an immediate emotional significance with the image (the path between the geniculate and the amygdala.) is severed and so it doesn't feel like his mother.
Finneas Gauge is another great, and classic, example. He was a man who used to work on laying rail roads, for which they had to use a gun to fire spikes into the ground. One day while packing the gunpowder, down it went off and shot a spike through his head, so that it came out the top and took out part of his brain. He was otherwise fine, but had begun to have childish tendencies, he was impetuous and crude where he had otherwise been a decent and sober man. This is because it had taken out his orbito-frontal cortex which is implicated in social emotions and as such ascribing social norms to behaviour, and without this, his behaviour was altered considerably.
There is a wealth of information on this, however the problem we encounter is just what consciousness is. What we have taken here is what is called a materialist stance, in that we believe that all conscious phenomena is reducible to the physical world; that if we could we could ascribe something physical which would create everything conscious.
The opposing view is what is called a dualist stance, where we believe that there are two worlds which are separate from each other. If we take it to its Cartesian extreme we believe that they aren't connected but by the grace of god and his influence on the events of reality as they appear. However there is much contention to most of Descartes ideas.
However a form of dualism may be necessary. This is because when we do reduce each of the sphere's down to their component parts, this is process would reveal whether there were precise correlates because there would have to be an exact amount of component parts for each. We have to remember that something can't create something else unless it's greater than or equal to it. As such if we take the component parts of consciousness and reduce them down, we are left with a progression which takes us to greater complexification. The more we reduce the gross aspects of phenomena down from, say vision, for example, then we are left with a wider and wider array of individual sensations: Every gradient on a spectrum of colour, tone, shade, the coordinates of movement, depth, position. Not just each of these but every considerable permutation of what already exists in our brain, should the right environmental stimuli provide us with a picture of it. This is just vision and not even hearing, every emotion, every reaction, belief, inference and so on and so forth.
The problem occurs however when we reduce the brain it its component parts. Neurons. Neurons are cells, as such they are composed of the same basic components as each other. They all have the same organelles, they still use DNA, and they all propagate electrical charge using the influx of ions across a membrane; the principle ions being sodium, chloride, calcium and potassium. Other factors for processing are neurotransmitters and the thickness of the axons. However, the problem becomes apparent when we think that different regions do undoubtedly, as was previously shown, cause different sensations. Knocking out the visual cortex, in our species and others, will knock out sight. However, the same basic bundle of proteins and electricity are responsible for the whole plurality of consciousness. This is impossible to believe, that an A minor, pride, the colour green, a hangover all have the same chemical composition. Though there are many neurotransmitters they are not concerned with content, there is not one neurotransmitter per type of perceptual category, they are how we process. AS far as content is concerned there is literally only the neurons.
Therefore, consciousness is not only not reducible to matter, it is GREATLY inadequate of explaining the complexity of consciousness on it's relatively simple components. So we must have to conclude that as far as we know it that consciousness must be some phenomena of the universe, which is wholly dependent upon matter and the greater and more integrated that matter has become the more powerful psychologically it has become. However, it is inadequate as an example of what consciousness actually is. It is far more complex and the most unique thing in the perceivable universe. It is anyone's guess for now. However quantum consciousness may shed light on answers eventually. Yet that is for another article, for now we can conclude that consciousness is something more complex and mysterious than anything in the universe. As ingrained within it is the paradoxical knowledge that all you can ever be and understand in any true way is that on the canvas of conscious being. Still, at the same time, it is the most elusive thing to pin down in the known universe as far as what has causes it. It could only be the product of the most complex system we know in the universe. But how they are connected and just what our conscious state IS is something which no one has yet been able to prove. Let me ask you this: What is the chemical composition of the colour green? That is not, what chemical's cause green, but what chemical components is, specifically, the very sensation of green? I guarantee not even the most advanced neurologist could tell you.