Monothematic delusions are false beliefs fixing on a single theme while the rest of a person’s reasoning remains largely intact. The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) classifies delusions as fixed beliefs that do not change in the face of contradictory evidence. Whether this classification is adequate has been contested, since delusions may lack the functional profile of belief. This review examines two conditions —Capgras syndrome, in which a familiar person is thought to have been replaced by an impostor, and clinical lycanthropy, in which a person believes they are turning into an animal — against four features attributed to belief: action-guidance, evidence-responsiveness, inferential integration, and affective coherence. The two differ in evidentiary depth, so conclusions are scoped accordingly and are not offered as representative of monothematic delusion as a class. Both guide action and are held with high conviction, yet both respond poorly to counter-evidence. Their inferential isolation is less secure than the philosophical literature assumes: the largest case series report that Capgras syndrome does not typically occur as a circumscribed delusion, weakening the central argument against its doxastic status. The evidence favors a modest doxastic account, on which delusions are beliefs that fail to discharge some characteristic functions. The DSM-5’s classification appears broadly correct; its limitation lies in a definition framed at the level of content and fixity, silent on the perceptual and affective anomalies that precede and sustain these states. Where an anomaly is ongoing, disputing the content addresses
the wrong target.