The concept of diagnosis is contingent on the concept of disease. Diagnosis is the name of a disease, just as, say, violet is the name of a flower. For example, the term "diabetes" names a type of abnormal glucose metabolism. ... Diseases (lesions) are facts of nature, whereas diagnoses (words) are artifacts constructed by human beings.
... Names, semanticists love to remind us, are not things. Manipulating things is difficult, sometimes impossible. Manipulating names is easy. We do it all the time. Violet may be the name of a flower, a color, a woman, or a street. Similarly a disease-sounding term may be the name of a pathological lesion or bodily malfunction, or the name of the malfunction of a car, computer, or economic system, or the behavior of an individual or a group. ... We cannot distinguish between the literal and metaphorical uses of the term "disease" unless we identify its root meaning, agree that it is the literal meaning of the word, and treat all other uses of it as figures of speech. In conformity with traditional practice, I take the root meaning of disease to be a bodily lesion, understood to include not only structural malfunctions but also deviations from normal physiology, such as elevated blood pressure or lowered white cell count. If we accept this definition, then the term "diagnosis," used literally, refers to and is the name of a disease, and used metaphorically, refers to and is the name of a nondisease.
... Historically, scientific medicine is based on the postmortem examination of the body. Recalling his early work as a neurologist, Freud proudly reminisced: "The fame of my diagnoses and their post-mortem confirmation brought me an influx of American physicians, to whom I lectured upon the patients in my department in a sort of pidgin-English." In scientific medicine, the pathological diagnosis always trumps the clinical diagnosis.
The use of diagnostic terms becomes problematic when the conditions they name are not disease but merely subjective, unverifiable complaints, referable to an individual's body, behaviors, or thoughts (communications). Psychopathology is diagnosed by finding unwanted behaviors in persons or by attributing such behaviors to them. For example, the term "kleptomania" is both a phenomenon and a name; diagnosis and disease are one and the same. Once "named," the diagnosis of a mental illness validates its own disease status. Psychopathology, unlike organic pathology, can change with the nosologychanging the name can convert disease into nondisease and vice versa (for example, homosexuality into civil right, smoking into nicotine dependence). Mental diseases are, a fortiori, diagnoses, not diseases.
Thomas Szasz, Pharmacracy: Medicine and Politics in America (2001:28-30)