Just finished reading A Very Short Introduction to Schizophrenia (I love this series, BTW) which has deepened my understanding of the neurological underpinnings of this disorder. I think I can better explain how it happens, for those interested.
There's a part of your brain that differentiates things internal from things external. It doesn't totally mute those things, but it does tone it done. That's the main thing that breaks down in schizophrenia, and depending on what other systems are affected determines how it manifests.
Say you move your arm. Simple maneuver, right? Now if someone else were to bump you and push your arm, you can tell the difference pretty easily. That's that section of your brain at work. But imagine a world in which you wanted to, say, pick up a pencil, and then instead of you picking up a pencil, someone else then took your hand and used it to pick up the pencil. People who experience this, after awhile, begin to think that someone else is controlling their movements. Even though their body is doing everything they want it to, because that section of the brain is broken, the one that tones down your own movements as "less important to pay attention to because you intended for it to happen" they don't feel in control. It may even digress to control of thoughts. Maybe someone else wanted them to move a certain way.
Did you think that thought or did someone else?
The same thing happens with speech. We all have an internal voice which is muted, and in addition, even the words we speak out loud are rated by our brains as slightly less attention-worthy than what others say. After all, we know what is going to come out of our mouths at least a fraction of a second before we say it (even if sometimes we, to much embarrassment, say things we didn't mean to express out loud). If the internal thoughts are where the major misfiring is occurring, you get auditory hallucinations, and because they are so "close to home" they may actually be louder than external voices. In a few instances, it's been known for schizophrenics to actually mutter the hallucinatory words to themselves, without being aware of it, and then answer those same "voices". And of course, since you can hear "them" too (because the person just said it), obviously those voices are real!
Now, it is entirely possible to hallucinate sound without being schizophrenic. They are the most common type of hallucination. ANYONE who experiences extreme anxiety, after awhile, will naturally begin to have this happen. I don't know why; I know it has something to do with the excess cortisol. But for those people, when the anxiety drops those hallucinations usually dissipate. Not so in schizophrenia.
Visual hallucinations are pretty much the end of the road. Usually by that time the whole system has degraded. You know where your eyes go, but if I poked you in the eye your vision would be jogged. You know when you daydream, remember that we take in information with our eyes but "vision" actually occurs when our brains process this information. And by this time, if you are under delusions that someone is controlling you, or hearing voices, etc. nothing you imagine is bound to be very good.
It's of note that in quite a number of non-Western cultures, schizophrenics tend to hallucinate "helpful" voices. They experience soothing, calming, and guiding spirits, or a mix, such as spirits that protect them from another evil entity. Here in the so-called civilized world they are more often to think the government is listening to their thoughts and out to get them.
Your internal subconscious, the stuff of dreams, including all ridiculous dream logic, has now become your reality. You cannot tell it from reality, but your emotions flavor all that you encounter. If you are anxious or paranoid, the world is literally warped to that delusion. If you are safe, secure, surrounded by love and light, maybe it's not quite so scary. Imagine being trapped in your dreamworld and that has become your waking reality.
Descartes is famous for the line "I think, therefore I am" because it was all he could prove that was real. Obviously the mad experience a reality all their own, what I call "the Blue Car World". For those who haven't followed along, it's living in a world where there is a hallucinatory blue car parked in your way. No one else can see it and everyone else can pass through it as if it isn't there. You try and try, because you know it shouldn't be there, but all you get is smashed up knees from trying to walk into solid metal. Yet everyone continues to encourage you to try walking through the "obviously" imaginary car. Until, one day, frustrated and exhausted, you give in, decide to humor your delusions, and walk around the damn thing, because it's easier to just go on with your life with the understanding that for you, there's a blue car, THE Blue Car, and you don't get to live in anyone else's reality, and they don't have to deal with yours.
And if you're lucky it's just those Blue Cars are parked, rather than trying to run your down in the street, and they don't talk.
I once, as a teen, told a shrink that he couldn't prove to me that anything was real, not the room, not him. Descartes would have been proud. I was a very philosophical teen, used to say that no one could disprove that the universe wasn't created only moments ago by a highly advanced civilization, a "pack of wild Saturnites" pretty much on a whim. That they'd made it all, including our memories and aged dinosaur bones. Almost no one else around could wrap their heads around this; they'd say dumb things like "but I remember last year!" To my eyeroll and reply, "duh, if someone could actually do all of that don't you think they could create you with memories too?" Not that you can prove to me a memory anyway; eyewitnesses to crimes are full of false memories.
When our brains fail, our senses fail, and that part of our experience, that part of our reality no longer exists. When the entirely of our brain fails, we die, and this whole universe ceases to exist. If we were cut off from all our senses, only our thoughts would exist, only our thoughts would prove (to us, anyway) that we exist.
The shrink, by the way, put me down as psychotic for this, me and Descartes. I continue to think that shrink was kind of an asshole, but now I know better than to wax philosophical with shrinks.