text:  Professor of Neuropsychopharmacology at Imperial College London here in the UK. And he's here to help us navigate everything hallucinatory. Hello, David. Good afternoon. All right. So, of course, I'm going to start with the obvious question. What is a hallucination? Well, that's actually a very obvious question, but a very difficult question to answer. In the simple terms, a hallucination is a perception of something