Explain why the distinction between an overvalued idea and a delusion is described as not always sharp in practice, rather than as a clear dividing line.

The features that separate the two — onset, plausibility, and openness to outside opinion — are all matters of degree rather than presence-or-absence categories, and real patients can sit at any point along each of these continuous dimensions rather than falling cleanly on one side or the other. A belief might have gradual onset like an overvalued idea but be held with the rigid, unshakeable quality of a delusion, or might be implausible in content while the person remains genuinely open to discussing it. This is exactly why the book recommends careful, longitudinal assessment rather than a single-interview verdict — watching how the belief and the person's relationship to it develops over time gives a much clearer picture than trying to force an ambiguous presentation into one category or the other prematurely.