She knew it was a hallucination.

And he knew it was a hallucination.

Oliver Sacks What hallucination reveals about our minds

Another patient of mine had a different sort of hallucination.

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

I said, There is a special form of visual hallucination which may go with deteriorating vision or blindness.

There are other parts of the brain which are specifically involved with the recognition and hallucination of buildings and landscapes.

So I am going to be talking about hallucinations, and a particular sort of visual hallucination which I see among my patients.

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.