What’s the one thing everyone knows about delusions? That they’re false beliefs. Not so fast. Already we have two problems. First, there is much debate among philosophers about whether they are really beliefs (recently the linguist Dariusz Galinski has written a fascinating post about whether delusional utterances even always have to be propositional statements).
Additionally, it’s not clear that delusions always have to be false. An oft repeated sentiment in psychiatry is that even a true belief (“my wife is cheating on me”) could be delusional if held with the right (or, I suppose, wrong) sort of conviction.