A patient suddenly develops a fixed, false belief. Before revealing the answer, decide what distinguishes an autochthonous delusion from a delusional perception.

Ask whether a real, external perceptual event triggered the belief, or whether the belief simply appeared, fully formed, with no such trigger at all. An autochthonous delusion differs from a delusional perception specifically by the absence of a real, external perceptual event that the new meaning is attached to — the delusion contains no such second, external step, arising instead directly and completely in the patient's mind. A delusional perception, by contrast, requires exactly this: a normally perceived, real stimulus to which a sudden, delusional meaning becomes attached. Both are forms of primary delusional experience, arising with no other morbid experience behind them, but only one is anchored to an actual perceptual trigger.