A 19-year-old man under heavy exam pressure and sleeping poorly describes feeling "like I'm on autopilot, watching myself from a few feet behind my own head" for the past several days. He finds it unpleasant and says he knows, rationally, that this is just how it feels rather than what's actually happening. Before revealing the answer: what is this, and what feature of his account rules out a nihilistic delusion?
This is depersonalisation. The feature that rules out a nihilistic delusion is his retained insight — he explicitly knows this is just how it feels, not an actual belief that he has ceased to exist or isn't real. That distinction between a distressing feeling and a fixed false belief is exactly what separates the two.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 7, 'Disorders of the Experience of Self'.