The theory was first written by Gilbert Harman, and then popularised by Descartes in his 'Evil Demon' thought experiment. In simple terms, it says that your brain could be somewhere else, manipulated by a super-computer that gives you sensory experiences. In other words, the reality you perceive isn't the real one, it is created by your brain, which isn't necessarily in your head.
Descartes' thought experiment says that we live in a simulation controlled by an evil demon who can easily determine what we perceive, and therefore make whatever it decides correct or incorrect.
'I am not right or wrong but thinking makes it so' ~ William Shakespear
What do you think the demon would make out of the Brain in a Vat theory? Of course he would prove it wrong! Otherwise he will be seen. Read a bit about it and almost all you will find is arguments proving it wrong.
But what if... you weren't really here? What if this *really* was a simulation?
Why is that so scary? The demon makes it so! And that same demon also makes sure we never look within, but focus on solving problems within the world (that doesn't really exist). All of the news we are exposed to, the dangers, the deaths, the competition - these things massively limit our perceptions of ourselves.
But awakening, enlightenment, salvation - all of these terms point to the same thing - that there is a reality much more beautiful than anything you can imagine, beyond this world, and beyond what you currently know.
'All I know is that I know nothing' ~Socrates
The demon manipulates reality so that we think we know what we are doing (if you are honest with yourself, you will see you haven't got a clue!), and makes not knowing the scariest thing. 'Darkness' is often associated to the unknown, and being 'in the dark' is perceived as uncomfortable and fearful.
I think knowing is scary! It traps the infinite into a tiny, tiny box.
I can already hear you ask: how do I know that I'm not manipulated by the demon? Here's how: there is a reality that just is, that can't be changed. The demon can only operate within the realms of experiences that the vat can create, not within the reality that created the vat.
The last realisation is that there's never been a vat nor a world.
If you have some time and want to experience an aspect of the Brain on a Vat theory, you can do the exercises from this webpage (takes about 20min):
Imagine that as soon as you are born and you develop the ability to think, your thoughts are like passengers boarding on a plane & going through a security check.
Most of them go through the check without a problem, and in that sense are 'safe thoughts'. They simply go through and out of your mind without any sort of identification. Others, on the other hand, are judged as 'bad', 'wrong' or 'unacceptable' thoughts.
These go to jail.
As we grow older, the thoughts that we 'jail' increase in number because, based on the person we start believing we are, we form a belief system of 'right' and 'wrong', of what deserves punishment and what doesn't.
But the more thoughts we imprison, the more chains we tie around ourselves.
Few know that the plane itself is an illusion, let alone the security check and the prisoners.
No thought is ever true.
No event can last in time unless you carry it in your memory.
No action is unforgivable.
Nothing after 'I am' means anything.
The path to Freedom, therefore, is to notice the imprisoned thoughts, and let them go. You don't need to dwell on them, to delve into them too much, to see why they happened, or to make many prisoners out of one. All you need to do is notice that you're holding one, open the cell door, and let it go.
And that is where faith comes into the equation. Without having faith that something beautiful, peaceful, joyful - just something worth going for - is on the other side of your fears, you
wouldn't dare to face them. You will always hold them as true, make them a part of who you are, and let these fears guide your actions.
This video illustrates this point powerfully-Will Smith on Skydiving.
There is only ego and non-ego, and all that is ego is illusion. The only way to remain in the peace of everything that is outside of ego (that happens to be infinite & eternal bliss) is to notice all the fears that compose the ego. And all that is ego is a fear in disguise. Hence the stronger your faith, the deeper in your self-made prison you can go to uncover and let go of deep-rooted fears.
Do not believe in yourself, because that is believing in an illusion and asking for pain. Believe that there is something much bigger and much more beautiful than your ego self - your True Identity - and take the necessary steps to Be it.
When you are on a path - any path - it is very easy to fall into the trap of wanting to go back to a previous, happier time. But you don't really want that - even if it was possible, why would you want to go back a few steps, and then re-live what you are currently living?