Herman Alexander Diels (1848-1922) published Doxographi Graeci in 1879. In many ways this work established the critical discipline of doxography - the editing, cataloguing, and analysing of extracts of extant classical texts that contain references to the ideas and arguments of lost authors and schools. In Doxographi Graeci Diels analyses passages from the extant work of authors such as Plutarch, Arius Didymus, Diogenes Laertius, Ps-Plutarch, Hippolytus, Ps-Galen, Stobaeus, Theodoret and Eusebius and uses them to uncover information about the Presocratic philosophers and schools whose written treatises are no longer extant. Diels’ method of filiation of extant sources, based on the critical methods of his teacher, Herman Karl Usener (1834-1905), allowed critical judgements to be made regarding the reliability and usefulness of extant authors and their references. Diels’ magisterial work represented a profound breakthrough in the study of the Presocratic philosophers. It is a monument of classical scholarship.