Psychosis is a mental state where people experience a 'different' world. If, as clinical psychiatry and neuroscience suggests, it is our 'default mental state' why isn't everyone psychotic? Psychosis does not arise de novo; external sensory input and cognition actively inhibit its expression. It is important to understand: how thin the boundary is between sanity and madness and what leads from one to the other; and to appreciate the frailty of rational thought.