Oliver Sacks What hallucination reveals about our minds

But she was frightened. She'd lived 95 years and she'd never had a hallucination before.

So if the world is not as it seems and we see things as we are, then what we call reality is a kind of hallucination happening inside here. It's a waking dream, and understanding that that is what we actually exist in is one of the biggest epistemological barriers in human history.