But it is pride that argues you have come into a world quite separate from yourself, impervious to what you think, and quite apart from what you chance to think it is. There is no world! This is the central thought the course attempts to teach. Not everyone is ready to accept it, and each one must go as far as he can let himself be led along the road to truth. He will return and go still farther, or perhaps step back a while and then return again. (ACIM:W-132.6, highlighting added)
ACIM's theology is also grounded in the idea that people's experience of the world is a cognitive mistake or error. This is very different than the best of Indian Advaita, which teaches this same idea as a preliminary doctrine but ultimately counsels that the world is a dream or illusion emanated by God (Brahman), not by the ego, and that it is an emanation out of pure Divine playfulness or lila (pronounced “leela”) and loving Grace, not a "mistake."
You who have tried to learn what you do not want should take heart, for although the curriculum you set yourself is depressing indeed, it is merely ridiculous if you look at it. Is it possible that the way to achieve a goal is not to attain it? Resign now as your own teacher. This resignation will not lead to depression. It is merely the result of an honest appraisal of what you have taught yourself, and of the learning outcomes that have resulted. Under the proper learning conditions, which you can neither provide nor understand, you will become an excellent learner and an excellent teacher. But it is not so yet, and will not be so until the whole learning situation as you have set it up is reversed. (ACIM:T-12.V.8, highlighting added)
Projection makes perception. The world you see is what you gave it, nothing more than that. But though it is no more than that, it is not less. Therefore, to you it is important. It is the witness to your state of mind, the outside picture of an inward condition. As a man thinketh, so does he perceive. Therefore, seek not to change the world, but choose to change your mind about the world. Perception is a result and not a cause. And that is why order of difficulty in miracles is meaningless. Everything looked upon with vision is healed and holy. Nothing perceived without it means anything. And where there is no meaning, there is chaos. (ACIM:T-21.in.1, highlighting added)