He is now known for a collection of quotations from and reports about Presocratic philosophers. This work, entitledDie Fragmente der Vorsokratiker, is still widely used by scholars. It was first published in 1903, was later revised and expanded three times by Diels, and was finally revised in a 5th edition by Walther Kranz and again in a sixth edition. It consists of three volumes that present, for each of the Presocratics, both quotations from their works transmitted by later writers, and secondary-source material known as testimonia. Based on Diels' enumeration of the fragments, the testimonia in the Diels collection are known as the "A-fragments", while the quotations from the Presocratics are known as the "B-fragments". Diels's method of labeling the fragments has become the standard way of referring to the works of the Presocratics. For example, what is thought to be the introductory section of Parmenides' poem on the "Ways of Truth and Opinion" was quoted by Sextus Empiricus and Simplicius; in Diels–Kranz this is labeled as fragment 28B1 — i.e., chapter 28, section B, fragment 1. The "28" stands for Parmenides, the "B" indicates that it is a quotation, and the "1" means that it is the first quotation in Diels' ordering of quotations of Parmenides. On the other hand, the beginning of Plato's account of an alleged visit of Parmenides and Zeno to Athens is labeled by Diels as fragment 29A11. "29" stands for Zeno, since this particular passage in Plato has more directly to do with Zeno than Parmenides; the "A" indicates that it is a "testimonium", a story about the philosopher in question, not a quotation; and the "11" means that it is the 11th testimonium about Zeno in Diels. The ordering of Presocratics in Diels is roughly chronological ; the numbering of the fragments themselves, within each chapter, is determined generally by the alphabetic order of the names of the sources. The usual way of citing fragments in Diels' edition is to append "Diels–Kranz" or the letters "DK" to the fragment-number; so for example "28B1 Diels–Kranz" or "28B1 DK". Often, a commentator will refer to a fragment in Diels–Kranz in a more abbreviated form. For example, one may refer to 28B1 as simply "Parmenides, fragment 1". In spite of the respect paid to Diels' monumental work, there is ongoing controversy among scholars over the details of his arrangement of the fragments. For example, some fragments categorized by Diels as quotations are thought by some scholars to be in reality only paraphrases or explanations of the Presocratic work in question. Also, Diels–Kranz does not of course include fragments discovered since its publication, such as fragments from the Strasbourg papyrus, which preserves for us pieces of Empedocles' poetry never before known in modern times. An English translation or paraphrase of each of the B-fragments in Diels–Kranz may be found in Kathleen Freeman's Ancilla to the Pre-Socratic Philosophers, though it is based on the fifth edition of Diels–Kranz, whose numbering of fragments is somewhat different from later editions.
