In this article, the author talks about Galen, a Greek physician who is known to psychologists largely for his personality theory of the four temperaments, and his method of diagnosing the basis of one of his patients' symptoms using a form of single-subject reversal design long before its formal description. Galen's method to diagnose the cause of a patient's symptoms is unique in terms of antiquity, design, and purpose. In working with a patient, he developed and applied a version of the single-subject reversal design. Here, the author discusses the history of systematic single-subject research and provides examples of early single-subject research and application. He also discusses Galen's implementation of a reversal design and the development and application of a version of a single-subject reversal design.