 How might mental disorders be distinguished from non-disordered mental states and conditions? Depression and anxiety, the schizophrenias, obsessions, compulsions and the like are core instances of mental disorder, but psychiatric classification acknowledges many other conditions as well. From such a large and apparently heterogeneous collection, the broad category may best be judged a family resemblance one. Indeed, through a different historical or cultural lens, the assembled variations known today as mental disorders would likely appear incoherently arbitrary and diverse. Yet, spurred by essentialist expectations (and out of concern about the unwarranted “medicalization” of normal states and behavior), the quest for a unifying conception of mental illness or disorder has persisted within philosophical research. Characteristic features have been identified, including disunity, irrationality, the presence of suffering and disability, and several forms of dysfunction. Formal analytic definitions have also been proposed, and are illustrated in  Section 4.4.