The fact that the human brain is as complex as the universe is not a secret for anyone. We are on twenty-four hours a day and seven days a week, from the moment we are born to the moment we are seven feet underground. Thinking, doing, calculating, dreaming... Our minds speak in a way only ourselves understand, reflecting on our bodies and health. As we never stop, there are usually way more than just one thing going on in our brains - running in the background, lurking. It can be just simply other thoughts.. like what you are having for breakfast or what was your math homework - or they could be voices.
You can't deny that, at least sometimes, you hear these voices. They can be talking about random stuff - for example, don't you catch yourself thinking about things you don't know where they came from, as in 'what goat`s stomachs look like?' - or giving you not so pleasant ideas - like how you could easily cut open your pain-in-the-ass boss'  throat when you are with that sharp knife in hands, slicing your Thai chicken during lunch break? - or putting you down - for instance listing all the mistakes you've ever made in your life and that you are worthless because of them, because of your past, because of your looks?.
You would be lying if you said nothing like this has had happened to you.
But I'm not interested proving how honest you are being. I presented you some immutable facts, which means that they aren't going to change because of you or your beliefs. What I'm interested in is actually how these voices are in our minds.
A curious fact is that people whose have louder voices on their heads are classified with as.. not so sane. I'm being smooth about it - because well, other people refer to them as lunatics, scientists, as schizophrenics. I'm not saying they aren't right, but I'm not saying either that they are correct. Rampage killers many times said that they did what they did because of the voices - but so did the guys who have written the bible. I'm just saying, And I mean, yeah, there is scientific evidence to this disease but... it's not completely understood.
Because well, back to my point, the human brain is far too complex to such 'simple' answers like genetics and drug abuse. Look, I am no scientist and yes, I might not have a thesis in such subject, so just take me easy. Just understand where I'm coming from. I didn't say they aren't right, I just think there is more to it... In another level.
In a spiritual level.
This is the part you probably will stop taking me seriously. That's why I will tell you my story, but differently to what I said about the voices in your head existing, this is not immutable - just because I can't prove it to you any way other than with my words. So I understand if you think it's bullshit - I wouldn't believe me either.

There were always there. You know how that is like because if you are reading this, it's a sign you have a mind too. It just comes so naturally for us we don't think about it.
For me, it all happened like this.
It was a rainy morning and my alarm rang, waking me up for another Monday. My nerves all screamed, tired and hungover.
'Dammit, why don't you just turn this thing off and kill work?' a hoarse girly voice asked.
'Maybe because she won't be able to pay the rent? You know well an apartment in Mainden Boulevard is not cheap.' replied the serious British one.
'And she needs to save up for that Chanel slutty dress for her date night with Andrew.' a high-pitched 14-year-old said.
Giggles.
This all would be fine... If I weren't completely alone in my room.