"Any split in mind must involve a rejection of part of it, and this is the belief in separation."
What is a WHOLE mind? A whole mind is at least as large as the universe. As Jesus states, "both heaven and earth are in your mind" and "you maintain the world in your mind with your thoughts". Realize that your mind has to be LARGER than planet Earth in order for Earth to be in your mind. It's not appropriate to try to imagine that your tiny little self inside your body is projecting this entire world. In fact, "the world" is this entire universe, and so let's say a WHOLE mind is large enough to contain THE WHOLE of everything.
A SPLIT mind, is a mind which believes in separation. The split is like a division, which literally seems to divide your mind into TWO halves. The division is upheld by some kind of "wall of sin", which comes between the two halves. This wall is made of illusions. This wall IS THE WORLD YOU SEE and the world you made. It is the physical matter of this universe. "The world is a picture of the crucifixion of God's son."
So if your whole mind is larger than the universe, what does a split mind mean? It means you place a wall WITHIN the universe, to separate off one part of your mind from another. What is that wall? It's all physical matter, actually, but more symbolically it is your BODY. Your body is the wall of flesh that you surround a part of your mind with.
The body is a device for separation. It has two functions. It keeps part of your mind INSIDE, and keeps the other part of your mind OUTSIDE. Remember you were WHOLE before, you were unlimited in size, and now you've SECTIONED OFF only a PART of your mind and placed it "inside the body", so all the stuff outside the body is THE REST OF YOUR MIND. You, who seem to be in a body, are only PART OF YOURSELF.
You're used to thinking in terms of everything applying only to the mind that seems to be inside the body, as if that is "you". And as if everything outside the body is "not you". And THEN you think that the whole of ACIM applies only to this "is me" that's inside the body. But it DOES NOT! THAT is part of the illusion of the split. It's a state of massive disassociation. In fact this is also part of how you pretend to be a figure in a dream.
When you dream, you dream an entire ENVIRONMENT. A world. But then you divide up that world, you fence off part of it as a body, and then place a fragment of the mind seemingly INSIDE that body. That part of mind now peers out through the body's eyes upon the environment, but because it is a DISASSOCIATED part, it does not believe that the environment is part of ITSELF. It thinks it is "not me". And it also does not believe it PUT IT THERE.
It doesn't see itself as the DREAMING mind, only as PART of the dream. And you in your human body right now experience exactly the same disassociation - you do not believe or perceive that this ENTIRE WORLD is inside your mind, or that you are dreaming it from a larger mind. You think you are ONLY the thing "inside" the body. This is part of being a split mind.
When you have a split mind, one half of your mind disassociates from the other. The ego splits off some parts of you and labels them as sinful, guilty, wrong, bad, and tries to destroy them. It does this in order to try to only preserve the appearance of being "good" and successful and perfect and innocent. In psychology terms, this is the PERSONA, which is ONE HALF of your mind and yet believes it is ALL of yourself. This is to do with where the course talks about the sunbeam believing it is the sun. This split off part of mind thinks it is ALL of you.
The parts of your mind that it disassociates FROM, and keeps believing is "not myself", it sees as "outside of me". Separate from me. And the wall of flesh that is the body acts as the divider between your persona and the REST of you. The rest of you is OUTSIDE the body. Due to disassociation you do not believe that the outside world is you AT ALL, or has anything to do with you. That you didn't put it there nor is it the location of your SELF. But it is, and you did put it there. You needed to make a world outside the body in order to be able to have a world to disassociate from.
What then happens is, within your mind, within the whole of your mind, there are now two seeming selves. There is the part that you think is all of you, and there is another part that you think is not you at all. That "not me" part is hidden behind a wall of denial, which is a block to awareness, which makes the REST of your mind unconscious. Since your persona self sees the REST of your mind as "outside of me", or separate from itself, it regards the "shadow" as OUTSIDE the persona. Now, the BODY IS the persona. Within the dream, it represents this fake "face of innocence", and everything OUTSIDE the body is YOUR UNCONSCIOUS MIND.
Within your mind, then, since "ideas leave not their source", everything that your mind can ever experience HAS to be within your mind, otherwise the mind cannot be aware of it. So that means your "inside self" is in your mind, your BODY is in your mind, and the outside WORLD is in your mind. And your mind is larger than all of them.
Within your mind, you will experience that there is war happening between one half of you and the other half. There will be a perception of "your self" (one half) as "the innocent victim", and it will seem that "the not me" self (outside "you") is the sinful victimizer. Your will is going to be divided against itself, because both parts of your mind are not in agreement. They conflict. And so the inner you, your persona, believes that it is under attack by "an alien will", that it THINKS is "outside of it", but which really is just another part of the mind.
You have this fully INTERNAL war with yourself, between these two parts, but because of the disassociation, your inner part BELIEVES that the "other me" is OUTSIDE the body. That your enemy is out there and is "not myself". So it is seen as "beyond yourself". And since your idea of yourself is confined within the body, that means you think it's outside the body. Even though it's in your mind.
It then becomes very EASY, to "project" or map this dynamic of a split mind, ONTO The world. Whereby you seem to "project" your sin onto your brother. But it's not really projection as such. It's more like, your mind actually believes that YOUR sinful parts are separated off from your "self" and are SEEN outside of you, beyond the body, and so you see them IN the world as if they are external. Your mind locates them externally because that seems to make sense in terms of them being disassociated from what's inside your body.
And so if you're believing/perceiving that the sinful part of YOUR mind, your "inner enemy", the "not me self", is OUTSIDE of your body, it's VERY VERY easy to assign or associate that with OTHER BROTHERS in the dream, whose bodies are walking around the same environment having split minds. Since you already believe "the sin is not in me, it's outside me", and since you believe" I am inside the body", you'll easily believe "the sinners are outside the body". And so there's a very small step from there to actually being deluded that some actual OTHER brother is the separate self who is sinful. And so you attach your split mind TO him and accuse him of your sins.
But in truth, all of this is really taking place inside your larger mind. Both what is inside your body and what is outside your body is WITHIN the confines of your larger mind. The innocent seeming victim that you see your PARTIAL self as, and the external victimizer evil sinful menace, are BOTH parts of YOUR mind. And this is what the secret of salvation is designed to show you. To merely look and realize that your brother has NOTHING to do with you being attacked, there is actually ANOTHER PART OF YOUR MIND ATTACKING THIS PART OF YOUR MIND. You are at war with yourself, and you literally are your enemy.
ACIM: "The secret of salvation is but this: That YOU are doing this UNTO YOURSELF. No matter what the form of the attack, this STILL is true. Whoever takes the role of enemy and of attacker, STILL is this the truth. Whatever seems to be the cause of any pain and suffering you feel, this is STILL true." - that external attacker IS PART OF YOUR MIND.
So that means you are ACTING the part of being the victim, AND you are acting the part of the victimizer. You are BOTH. You are the inside and the outside. And until you can recognize that what you see in another is part of your own larger mind, you won't stop projecting it onto them. When you own it, and take responsibility for it, that means realizing that this "separate enemy" is actually a character IN your larger mind, seeming to interact with this other persona character IN your mind, and BOTH of them are like puppets that your LARGER mind is operating. Your larger mind is pitting parts of ITSELF against each other.
What makes it SEEM as if there are two identities or characters, is illusions. Illusions present a picture of images and appearances and behaviors that SEEM to make it look like there are TWO distinct persons, two wills, two minds, to selves. But the challenge you have, is to OVERLOOK (forgive) those illusions. When you do so, you should come to realize that your larger mind is responsible for playing the roles of BOTH of those two characters. That who YOU are, as a whole, is WITHIN BOTH of them. If you can see that YOU are literally playing BOTH sides and ARE both of them, that "unity" will transcend the illusions, and it will dissolve the split.
You won't be able to believe in the split, or in being only one of the two halves of the split, if you can see how your WHOLE mind is in BOTH sides. It's like owning up to being one who has been pulling all the strings, MAKING THE DREAM ENVIRONMENT, and causing all things to happen. That means moving UPWARDS, from a position of "I am only this little self in a body" to " I am the whole of everything everywhere". And it is from that larger WHOLE mind that you are the DREAMER OF THE WORLD.
ACIM: "For you would not react at all to figures in a dream (as a smaller part of mind) you knew that YOU (your larger mind) were dreaming. Let them be as hateful and as vicious as they may, they COULD have no effect on you (your larger self), unless you failed to recognize it is YOUR (whole mind's) dream. This single lesson learned will set you free from suffering, WHATEVER form it takes." - this is a shift in identity from "I am a PART of mind in a body" to " I am ALL of mind everywhere."
If your mind transcends the separation, ie you stop believing illusions of separation, you will become aware of your wholeness. That's a transition from being two selves on a lower level, to being one unified self on a higher level.
What that also means in terms of the fact that your mind is the size of the universe, is that you should eventually heal the separation that is DIVIDING THE WORLD, which means removing the WALLS that the world is made of, which means undoing physical matter which is the illusion of separation, and that also means ERASING THE BODY. The body IS the device that acts to implement the split mind, WITHIN the larger scope of what you are.
In order NOT to have a split mind, your body HAS to be undone. It acts like a wall that is literally keeping part of you inside it and the rest of you outside it. It is made of illusions. "The body cannot stay without illusions". That means without the idea of being PART OF the world and not ALL of it, there cannot be a body there, representing the split in the mind and dividing and separating your universe-sized mind into two parts.
The body must vanish. It must be transcended and it must disappear. It is made of the illusion of separtion. "The body is a dream." And it is also the way by which the whole dream is maintained. There is no dream WITHOUT the body, because you have to MAKE a separation - a split in your larger mind, in order to go INTO dreaming. A split mind is a dreaming mind. And MAKING that split in your mind MAKES a body. It puts a wall there. Therefore when this dream of "opposition" and "competition" and "taking sides" is ironed out and ends, so too does the body wall disappear, and so too does the dream disappear, and that includes the Earth.
In effect, the undoing of the belief in separation is the undoing of the WALL within your split mind which IS the body, and this cancels out the differences between the inner and outer parts of mind, makes the mind WHOLE, and a whole mind CANNOT BE ASLEEP DREAMING. A whole mind cannot have a body. And a whole mind cannot be IN a dream world. The dream world only lasts so long as there is an illusion of separation, which depends ENTIRELY on having a split in your universe-sized mind. And this is also why "you cannot remain in this world, with this knowledge." Knowledge collapses spacetime.
As you return to being whole, you will also return to a state of mind where what is inside you is also seen outside you. ie it is everywhere the same. That means seeing the holy spirit in all your brothers because he is in your mind. Seeing God "in the world" because God is in your mind. Your wholeness reunites you with THE REST OF YOURSELF WHICH IS IN YOUR BROTHER. Your wholeness rejoins and reconnects YOUR mind, to the REST of your mind, which is currently believed to be "not inside you". Your sense of SELF, therefore, has to expand from "I am that which is inside the body" to "I AM EVERYWHERE". That results in oneness with all brothers.
To be whole is to be restored to an awareness that YOU are everywhere, you are larger than the universe, you are unlimited and infinite and immortal, you are not partial, you are not located in space or time, and you are beyond the dream of death. You are beyond the world. The world was merely YOUR SPLIT MIND, while you were dreaming. It's forms were YOUR SPLITS. Its walls were your illusions of disassociation. Without you having a split mind there can be no dream world. Waking up is waking up to WHOLENESS. Wholeness is reality. In wholeness, there is no world and there are no bodies and no walls.
You are unlimited. You made this entire world as a result of splitting your mind. When your mind is whole, the world will disappear. You are a huge, unlimited, spiritual being and this TINY world is nothing against your magnitude.