The organs of thought sometimes go wrong. What is a delusion? What kinds of thoughts are delusions? How can they be explained? ‘Thought gone wrong’ acknowledges that there are no solid answers to these questions but explores how far we have come in trying to find the answers. There are two types of delusion: monothematic (restricted to a single theme) and polythematic (encompassing a broad array of topics). Delusions can take on the nature of beliefs. What is the relationship between delusional thought and ordinary thought? We can learn a lot about thought by studying delusions but an obstacle to this is that we lack a good model of non-pathological belief formation.