Should 'delusion' denote any fixed false belief, rather than a uniquely psychotic phenomenon?

Delusion denotes a psychotic phenomenon in DSM and in the academic literature, yet we have been unable to craft a definition of delusion that rules in all beliefs considered psychotic and rules out all beliefs considered nonpsychotic. In fact, most of the problems of defining delusion stem directly from framing delusion as a psychotic phenomenon, which is hard to square with the reality that the cardinal epistemic features of delusions – falsity and fixity – are not unique to psychotic beliefs. Moreover, there is no precise, consensual definition of psychosis from which to extrapolate criteria by which a fixed false belief demonstrates psychotic provenance. The theoretical implications of the falsity and fixity of many nonpsychotic beliefs, along with the lack of a precise enough definition of psychosis, arguably justify redefining delusion as "a fixed false belief [regardless of etiology], held despite clear and reasonable contradictory evidence regarding its veracity", and using specifiers to indicate putative cause of the belief (psychotic delusion, conspiracy delusion, cult delusion, body dysmorphic disorder delusion, etc.). The proposed definition revises the DSM-5 definition by reinstating falsity as a criterion (which I argue is necessary and justifiable despite the difficulties it presents), and by changing the definition of fixity, for reasons discussed, from “not amenable to change in light of conflicting evidence,” to another characterization of fixity in DSM-5 (slightly altered), “held despite clear and reasonable contradictory evidence regarding its veracity” (see p. 87). The proposed definition of delusion eliminates the implication that delusions are necessarily psychosis-driven or otherwise pathological. Using delusion to denote any fixed false belief would bring epistemic value. It would underscore that falsity and fixity are not unique to psychotic beliefs, correcting an overfocus on falsity and fixity that hampers the search for unique markers of psychotic provenance. A comparison of various fixed false beliefs suggests they share some dimensions of etiology (e.g., misleading input, information processing biases, failure of belief evaluation); and within those dimensions, have both similarities and differences. A generic definition would further the discovery of such similarities and differences, serving to refine the concept of psychosis by caveating assumptions about psychotic processes that were based in research conflating delusions with psychosis. The fact that psychotic cause is an implicit, de facto criterion of delusion creates theoretical and diagnostic problems regarding true and/or unfixed beliefs with psychotic origin, and fixed, false beliefs with non-psychotic origin. Uncoupling delusion and psychosis would eliminate such problems, e.g., whether a true belief, or a religious belief, could be considered a delusion. Delusions would still be viewed as a key feature of psychosis, but not specific to psychosis. Instead of continuing to ask what it is about delusions that is psychotic, the question would become: what is it about psychosis that is conducive to delusions? Finally, this change in usage is arguably just below the surface, waiting to crystallize.