Daniel Dennett is famous (even notorious) for his thinking about consciousness. He is dismissive of ways of thinking that presents consciousness as a "Cartesian Theatre". The Cartesian Theatre is an outgrowth of Rene Descartes' investigation of the mind.
We remember Descartes for his famous conclusion, “I think, therefore I am”. This came at the end of a skeptical investigation where he showed that he could doubt the existence of just about everything, from physical reality to his own body. What he eventually found that he couldn't doubt was that he was sitting there doubting. This posed a problem for Descartes because he was well versed in the anatomical knowledge of his time and also knew about the various automatons that were being shown (mechanical men controlled by levers and pulleys and wires). He (correctly it turns out) declared that the material stuff he knew about could not enable his ability to think. From there, in a world where everyone already believed in a spiritual world with heaven and hell and God, it was easy to leap to the conclusion that thinking, and all mental activity, took place in a mental dimension that was apart from the physical dimension. This is a dualism whereby the universe is composed of physical stuff and mental stuff. It was just obvious to Descartes and others of his time (for example Liebniz) that physical stuff could not possibly be conscious.
Somehow, via our sense perceptions physical reality is presented to mental reality. Dennett uses the metaphor of a theatre for that presentation. Information about reality is presented in a Cartesian Theatre to our minds for our consideration and then our minds tell our bodies what to do. Nobody ever gave a good explanation about how that could possibly happen, but in a world where so much was explained by magic that wasn't much of a problem. As science developed over the centuries though explanations that depend on magic or the supernatural are rejected out of hand. This eventually posed a fatal problem for dualism since dualism proposed a non-physical something changing the amount of energy in reality – which violates the laws of Thermodynamics which says that physical energy is never either created or destroyed; it just changes form. Because of this fatal problem most philosophers dropped any sort of formal allegiance to Cartesian Dualism.
Many contemporary philosophers still think it's just as obvious, though most don't give direct allegiance to dualism anymore. Dennett talks about this as Cartesian Gravity. Even though they know it's impossible many people fall under the influence of Descartes' idea that mental capacity can never be explained by knowledge of physical reality. Some talk about the Hard Problem whereby no matter how much we learn about brain processes we won't have even begun to scratch the surface of the mysteries of consciousness or perception. Others invoke philosophical zombies; creatures just like us down to the finest physical details, who act just like us, but by the hypothesis are not conscious. Many thinkers take it as just obvious that such creatures are conceivable and since they are conceivable that conscious is distinct from our physical body. Other thinkers talk about qualia. For instance – when I see something red that a 'quale' of red somehow shows up in the Cartesian Theatre.
Dennett deals with these ideas at some length to show why they are totally inadequate. A sort of barking up the wrong tree. But I won't go into the details of that here. I recommend his book Sweet Dreams: Philosophical Obstacles to a Science of Consciousness for a fairly thorough treatment of these ideas in layman's terms written in his lucid and amusing style. What I want to do now is leap to his conclusions about just what is involved when we think and perceive and feel and are conscious.
First, he explains that we have many interacting processes going on in the brain. For instance, seeing something red causes a certain set of neuronal processes to occur and he maintains that those processes don't cause (say) a qualia of red to be presented to consciousness as if presented on television. They ARE the experience. And they aren't qualia in the philosophic sense (a notoriously unexplained sense by the way). They ARE the experience and they inform the rest of the brain that something red is our field of vision. Other parts of the brain are recognizing octagons with white writing on them. Other parts of the brain are interpreting the writing. Other parts of the brain respond to all this information. If we are driving towards the STOP sign we step on the brake and stop. But if we are walking in a crosswalk we may keep walking knowing that the approaching car (seen to be slowing) will stop. And we respond differently again if the car isn't seen to be stopping. So what's happening is that of all the processes happening in the brain at any one moment, some processes come to influence the whole brain for a time.
That's what we perceive and are conscious of - Dennett has called this "fame in the brain" or "cerebral celebrity". It's not that the famous coalitions of processes are presented on a screen that we are conscious of; instead they just influence us directly. We aren't conscious of them. They ARE consciousness. And like fame, their influence can be transient. Once we are past the stop sign we have other thing to think about and respond to. Note that at any one moment there are very many such process going on in the brain. Dennett writes on page 161 of Sweet Dreams: “ What a theory of consciousness needs to explain is how some relatively few contents become to this political power, while most others evaporate into oblivion after doing their modest deeds in the ongoing projects of the brain. Why is THIS the task of a theory of consciousness? Because that is what conscious events DO. They hang around for a time [influencing us until other events take us over] . . . “
So why do one set of processes become consciousness and others don't? It depends. It depends on the current state of the brain. We can think of the brain as a sort of ecosystem that changes from moment to moment. And not all processes prosper in any one such ecosystem. For instance, if I'm shopping for a new computer I may not notice the wall of TV's right there in the same room and visa versa. Or if I'm talking on my cell phone while driving I may not notice subtle changes in the traffic pattern that portends trouble ahead. I admit I like Dennett's approach to consciousness - I find it satisfying and beautiful and useful.