How are we conscious of the physical world?
We are quite good at explaining macroscopic events in the external world. We can explain some events by reference to other events, and to laws that describe regular causal relations between events. Why did the vase fall off the table? Because the kids were running around, and the floor was shaking, and the vase was close to the edge, and we know things about gravity, and friction, and the transmission of forces, that can explain the vase falling in terms of the kids running around. We know these facts and these laws as the result of generations of study of the multifarous events that happen in the world.
There seems, also, to be a correlation between events in the external world and changes in the content of my subjective experience. Bright light falling on my open eyes causes an experience like this. When honey passes over my tongue, it causes an experience like this. But, unlike the case of causation in the external world, I have no general laws, no chains of explanation, that tell me how those external events cause these subjective feelings.
Now there are facts in the external world that are also unexplained. I know that gravity causes objects to attract each other with a force proportional to the masses of the bodies, and inversely proportional to the square of the distance between them. I don't know why that is, any more than I know why the vibrations in the air caused by a flute cause, when they strike my eardrum, an experience like this. In the case of gravity, though, I can imagine what an explanatory theory might be like, what sort of components it might have, how it might be testable, and so on. The same is not true of theories of consciousness; I cannot imagine what a theory would look like that adequately explained how causation crosses the barrier (so to speak) between the external world and inner experience.
There are complications here, of course. I do not mean to say that there are no laws or regularities that relate the external world to internal experience. I have never tasted shimp-flavored ice cream, but I can make a reasonably-accurate prediction of its subjective properties (of what it would taste like), based on past experience. I know roughly how it will feel if I go out in the cold, or if I put my finger in the candle flame. But this simple sort of prediction does not suffice as explanation, in the way that our elaborate and powerful physical (chemical, biological) theories suffice as explanations of the external world and its interrelations.
Even if we had a theory, and instrumentation, sufficiently powerful to build a machine that could make statements about my subjective experience that I would agree with ("you feel cold now", "you are melancholy", "you are thinking of Maud again"), the correlation we would have discovered would be unsatisfactory as explanation. It's true that whenever my neuron-state fits this and such complex description, I will in fact agree that I was thinking of Maud; but why? What lies behind that correlation?
Dennett or somebody has a possibly-interesting "mind reading machine" paper on this general subject. Look it up and see if it can be usefully cited.