Explain why the falseness of a delusional belief is not, by itself, what makes it delusional.

Falseness cannot be the defining feature because, by pure coincidence, a delusional belief can occasionally turn out to be true — a patient's fixed, unshakeable conviction that a partner is unfaithful, for instance, could happen to be accurate. What actually makes a belief delusional is the fixed, culturally out-of-keeping manner in which it is held: its imperviousness to counter-evidence, its origin outside any ordinary, understandable process of reasoning from the person's own background, and its being out of keeping with what that background would lead one to expect. Recognising this distinction matters because it keeps the diagnosis anchored in how a belief is held and arrived at, rather than in whether it happens to be objectively correct, which is not something a clinician can always establish in any case.