That may sound like a rather BIG question or maybe a rather REDUNDANT question. But, much of what A Course in Miracles is trying to teach has to do with your sense of,experience of and definition of reality.
Take, for example, these three lessons from ACIM workbook: lesson 7 "I see only the past"; lesson 15 "My thoughts are images that I have made" and lesson 32 "I have invented the world I see".
On a more humorous note, take Trudy's thoughts about reality from "The Search for Signs of Intelligent Life in the Universe" by Jane Wagner: "What is reality anyway? Nothin' but a collective hunch. My space chums think reality was once a primitive method of crowd control that got out of hand. In my view, it's absurdity dressed up in a three-piece business suit."
Back to mirrors! The trick is to keep seeing the relationship between your inner psyche and your outer world, between thoughts and emotions on the inside and experience on the outside. This is, of course, a very deep process because there is a whole lot more of you on the inside than the little bit that is going on on the outside. But in this way, you don't have to succumb to the traditional definition of outer reality, which is the little bit that is going on on the outside. You will have more options about how and what you experience.