The cave in this poem calls to mind Plato's famous image of prisoners who take shadows for reality, though the speaker puts a darker twist on it. The shapes on the wall are warped and circus-like, and the speaker first insists they aren't even shadows, then waves the whole thing off as "another trick of a sick mind." It's hard to tell whether that mind belongs to the speaker or to the world.
The tone shifts in the second half. The speaker grows gentler, almost consoling: don't be ashamed for believing what you see. Besides fight or flight, there is freezing, and belief is presented as a basic need. Trusting your eyes feels so natural that doubting it seems almost wrong. The mood is uneasy and hazy, with sounds that drift away from meaning, but also tender. The speaker seems to understand how easily people are fooled, and doesn't blame them for it.