Pivotal to best-practice medicine in the 21st century is the concept of differential diagnosis. What the classification of disease, nosology, is to pathology, so differential diagnosis is to the management of the individual patient. Doctor-patient interaction comprises a sequence that is a six-link chain comprising history-taking; examination; differential diagnosis; tests and investigations; provisional or definitive diagnosis; and finally, management. This chain, however abridged in detail, determines every medical encounter with every patient. This paradigm has gradually evolved since the time of recorded medical history. It had developed its present form, if not its name, in Western medicine and its derivatives, with the publication of Osler's The Principles and Practice of Medicine, in 1892. The term "differential diagnosis" first appeared in the first edition of Herbert French's book, An Index of Differential Diagnosis of Main Symptoms, published in 1913.