A patient describes a belief that seemed to appear suddenly, out of nowhere. Before revealing the answer, decide how you would explore whether this is an autochthonous delusion.

Ask the patient to describe exactly how and when the belief arose, specifically checking for any gradual development or identifiable external perceptual trigger, since a fully formed belief appearing abruptly with neither of these is the defining feature. Assess the degree of conviction, since the idea is experienced with the same total, unshakeable conviction as any other delusion. Bear in mind that this pattern is not diagnostic of any one condition by itself, since sudden ideas or "brain-waves" also occur, without any delusional quality, in people without mental illness — so weigh the content and context carefully, since bizarre or grandiose content carries more clinical weight than an ordinary sudden hunch or realisation.