A common model is the Cartesian Theatre named by Daniel Dennett, and this will do to start with.
You have a little copy of yourself, a homunculus, sitting inside your head looking at everything that is going on. Like a pilot looking out of the windows of your eyes.
Its a subtle misunderstanding of what is going on. It is true that we sit at the centre of the world due to perspective. A camera experiences this same central perspective due to how light enters the lens but no one thinks there is anyone sitting inside a camera. Yet the "inside" comes from this same perspective not from anything else. The consciousness that is aware of this is some thing else. But we aren't going to cover that here.
Firstly lets start with the homunculus sitting in their theatre looking at the sensory input and experiencing the outside world.
Now why does this homunculus think that this sensory input is "theirs"? If there was someone else in the cinema watching it all then they could share the experiences and we wouldn't think they were just mine.
Perhaps this issue becomes most poignant when our sensory data is numbingly bad pain. It becomes very important to our health that we see it for what it is.
If someone could enter the theatre and share our pain we would be saved from the worst part of pain that it makes us lonely and feel isolated from the people around us. We can feel envy or anger for people who do not have to suffer this. And we can get tired of the pain and just give up and want it gone leading to frustration and breakdown. If someone was with us experiencing it all, someone to hold our hand as we experience the pain together that would make the whole experience much more bearable.
And yet this miraculously is the actual situation! Sort of.
The point is that sitting in the chair in our cartesian theatre the pain is actually not ours, it is streaming in to us, but it is clearly "over there", it is not ours. The thing that makes it seem like ours is that only we are experiencing it.
But that is the uncanny thing. In this model we are necessarily the only one who can experience it. There are no doors no one else can enter. Necessarily. The doors are not only locked they don't exist.
If no one else can enter then why have the theatre at all? Only you can see the screen you may as well merge with the screen and get rid of the seats.
But we have established that we are also not the sensory data, it is over there and streaming in. We still feel that the world is happening to us.
How are both possible that we are not separate from the screen and at the same time the world seems to be over their and coming at us.
The answer was hinted above with the camera. The sense of "coming at us" is part of the experience! And the belief that it "belongs to us", or "is ours" is all part of the experience.
The experience itself is not ours.
Now this can only be clearly seen in jhana, States of complete concentration where we enter the screen of the cinema. Its often called subject and object merging, but I never found this description useful. Its more than the subject and the object explode from the screen creating the illusion of a difference between myself and the experience.
So in a nutshell with overwhelming pain the fact of the matter is that it is there, but its not happening to anyone. So we cannot actually be isolated in "our" pain, and we are not unlucky to be experiencing it, and its not unfair, and all those people who are not experiencing it are lucky, and we are sad. None of this is actually true. In the same way that there is no door for a second person can enter the theatre. In fact there are no doors for anyone to enter the theatre and experience the pain, not even us!!!!
This point was made before in previous posts. The existence of the pain IS THE OBSERVATION. You can't experience pain that hasn't been observed. But we make the mistake of "Naïve Realism" and think that the pain we see is real and lies outside us, and then it comes into contact with an observing self that then experiences it.
We can see our nose and body and we think the experience of things out there is still to happen to us. But to know about the things out there we must have already experienced them! That mountain far away must have already been experienced by us for it to get all the way over there! Things that have not been experienced simply are not there. So its pointless suggesting that the experience is happening over here in this body, when it must have already happened to be over there! This is the mistake.
Its like watching a cricket ball come towards us. We think the ball is "over there" and it comes ever closer and THEN it hits us. But light from the ball has already entered out eyes for us to see the ball. It is not "over there" really, it must in some way be in contact with us doe us to see it. What we "see" unfolding is a complex analysis of the situation made by our senses and brains already! We are effectively looking at a simulation of the ball. But that is the mistake, the "looking" lies in the creating the simulation in the first place. The looking is not a person sitting in a cinema looking at a screen, it is in making the film itself. We are the director and the film maker and the editor and by the time a final product exists the thing has already been seen. It is not then passed on to viewer! There is no point in showing it to a cinema going its already been seen.
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Now extremely important addendum. I speak about the rational side of enlightenment here. The Prajna. The seeing the world as it really is. But I invariably miss out the other half of this.
The removal of the person from the cinema and the presenting pain just as it is with no viewer is prajna and correct. But it is soulless. We might start to think that pain does not matter. But of course it is a very potent experience and a living thing is profoundly affected by pain. They may be able to let go of any personal involvement in the pain and not feel isolated, but it is still an actual intense experience.
What is missing from my account is compassion. The desire to help and fix pain is fundamental. Removing the personal aspect may seem to make suffering soulless. We all become stones. But quite the contrary it means that we approach pain the same regardless who or what is experiencing. By realising that no one is watching "my" pain frees me up to look at all pain the same! All pain becomes one pain, no longer fractured into "my" pain or "your" pain. Pain IS pain. This is the truth. When we are in pain we seek to fix that pain, not as a knee jerk reaction but as an acknowledgement that something is wrong for our body. We wish to help this body. And the same for any other body in need of help. Its is perfectly normal to seek to alleviate pain in any living thing. We do not need a huge metaphysics of souls an cinema goers being subjected to films. we can be quite normal and just see the world as it is. That is the true meaning of Prajna. Not the creating of more metaphysics but the liberation from layers of false view.
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So discussion of pain cannot exclude "sadism." How is it possible that the truth of pain can be reversed? Pain by definition is not pleasant. An ant faced with physical damage will not run towards destruction but will run away. The truth of pain is that we run away. Yet in the Colosseum in Rome the Romans put people deliberately in the path of pain and destruction. They rationalised this as giving them the opportunity to heroically fight for their freedom, but often things were so stacked against them it was nothing but a death sentence and a spectacle for the public.
So why would you place another person in the path of pain and destruction, and not respond to pain like you would if it was your own?
To get this mysterious situation you need live in a Cartesian Theatre. Inside your theatre you are "safe" and subjected only to the sensations that you get. With this view we become obsessed by what "our" cinema is playing. This is great when the film is good, but when the film is pain then "we" suffer. And we store that "unfairness" of suffering up as resentment. It means that when we see another suffering in "their" cinema there is a sense of justice and it makes more sense of all those times we have suffered alone in "our" cinema. The hardest people have suffered long and hard in their cinema, trapped before a wall playing only films of pain. That isolation and unfairness becomes a strong desire for justice and plays out as wanting others to suffer.
But as argued above its all a misunderstanding. There is no one separate and "watching" the film. The film IS the person. Usually the film is of someone experiencing something, and we identify with that person as "me" (e.g. watching a cricket ball flying towards the air to us) but we are not the person to whom the ball is flying, we are the actual experience itself. As said above: in the Cartesian Theatre view we are waiting for the ball to arrive because we see our self as the physical person standing in the field, when in actual fact we are fully engaged with the ball the whole time it is flying too! We are the whole film - otherwise how would "we" know the ball was coming? We are the person hearing the bat against ball, we are the person first seeing the ball, we are person waiting for the ball, we are the person that the ball arrives at! Yet in our mind the ball is over there and we are just the person standing waiting for it. Two quite different views of what is happening. The first absolutely correct, the second a story or myth used in daily life and this culture. Where did you play? I was a long off fielder. "Nice catch mate" people say etc. Ignoring the real complete event that actually unfolded.
Pain is pain it doesn't happen to anyone. Like no "one" actually catches a cricket ball. Ask any cricketer what they are thinking as they catch a ball. The very last thought is who is going to catch is. Or if they are thinking this they will probably drop the ball. To catch a ball you think about catching a ball. You spread yourself wide to be the person who sees the ball, who runs into position, who moves their hands in preparation, who judges where the ball we be, who grabs it in flight, feels the sting as it slaps into the hands, and who holds it like nothing else matters. They are all these things and all these people at once. Then they may look around at the recognition on other people and have the thought "wow I did it" "I am awesome" etc etc. Two quite different processes.
Likewise we can have pain without thinking who is having it. Indeed we never need to think about who is having it. Pain is just pain. Seen like this sadism cannot exist. It is just a mistake, or the dark shadow of years spent living a mistake. Forgiveness to let go of the wrongs done to us, humility to admit we need let go resentments, and compassion to wish an end to pain and suffering for our self and others. Complex and toxic mix is very easily possible, in fact its where most of us are to some extent. But in moments or peace more or less we can see where we are wrong.