This chapter describes the activity of medical diagnosis and its theoretical background as well as with some of the most frequently occurring misconceptions to be found in the literature on psychiatric diagnosis. It evaluates the early development of ‘official’ diagnostic criteria for inferring schizophrenia. The word diagnosis is derived from Greek and means to distinguish, to discern through perception. The term nomenclature refers to a list of approved terms for recording observations. The ideas that the subject-matter of medicine and psychiatry is disease or illness and that practitioners find diseases in people are extremely tenacious. The confusion evident in the literature appears to arise from a failure to make the crucial distinction between the original research which seeks to identify new patterns and infer new constructs, and the later activity of identifying new exemplars of these patterns. It is axiomatic that constructs which claim scientific status should be derived from observations about which there is a high level of agreement.