Explain why retained insight is described as the key feature separating a disturbance of awareness of self-activity from a nihilistic delusion.

In a disturbance of awareness of self-activity, the person retains awareness that this is a change in how things feel to them, rather than holding a fixed false belief about it — they still know, in some sense, that their thoughts and actions remain theirs, even though this no longer feels true from the inside. A nihilistic delusion crosses into a fixed, delusional conviction that something fundamental about the self genuinely does not exist, without that self-aware, "as if" quality. This distinction matters clinically because it places the presentation on different sides of a key phenomenological boundary: an unusual, distressing subjective experience that the person can still reflect on and describe as a change, versus a primary delusion held with full conviction, which in turn shapes how the presentation is understood and managed.