Two patients each hold a strange, fixed belief with great confidence. Before revealing the answer, decide what would help you tell a delusional perception apart from an overvalued idea.

Ask about onset and arguability. A delusional perception attaches suddenly and instantaneously to one specific, accurately perceived event, with total, non-negotiable conviction from the first moment. An overvalued idea, by contrast, tends to build up gradually over time and, however strongly held, is one the person could in principle be argued away from given enough reasoning — a delusional perception is not open to that kind of challenge at all.