As exemplified in the defining sentence on Belief (disambiguation), insofar as the truth of belief is expressed in sentential and propositional form we we are using the sense of belief-that rather than belief-in. Delusion arises when the truth value of the form is clearly nil.
Delusions are defined as beliefs in psychiatric diagnostic criteria[8](for example in the Diagnostic and Statistical Manual of Mental Disorders). Psychiatrist and historian G.E. Berrios has challenged the view that delusions are genuine beliefs and instead labels them as "empty speech acts," where affected persons are motivated to express false or bizarre belief statements due to an underlying psychological disturbance. However, the majority of mental health professionals and researchers treat delusions as if they were genuine beliefs.