In Chapter 30, Kaysen explains the feeling of dealing with mental illness using the following allegory:
Think of being in a train, next to another train, in a station. When the other train starts moving, you are convinced that your train is moving. The rattle of the other train feels like the rattle of your train, and you see your train leaving that other train behind. It can take a while—maybe even half a minute—before the second interpreter sorts through the first interpreter’s claim of movement and corrects it.... You can flit back and forth between these perceptions and experience a sort of mental vertigo. And if you do this, you are treading on the ground of craziness—a place where false impressions have all the hallmarks of reality.
Using the symbolic analogy of the illusion of motion on a train, Kaysen describes what one's relationship to reality can be like when they suffer from mental illness. She describes the experience of this common illusion as "treading on the ground of craziness" because, despite the fact that there is a true reality (that your train isn't moving), it is difficult to distinguish reality from the illusion because the illusion has certain elements of reality. Kaysen's allegory doesn't only help the reader understand "craziness" as a matter of pure perspective but also proves that all of us are capable of experiencing "craziness" (and quite regularly do).