Diogenes Laertius
Greek doxographer and biographer (3rd century AD), about whom very little is known with certainty — neither his home city nor the philosophical school to which he belonged — yet whose work Lives and Opinions of Eminent Philosophers constitutes, despite its limitations as a rigorous historical source, one of the most important documents to survive for reconstructing ancient Greek philosophy as a whole. Structured in ten books, the work offers brief biographies, anecdotes often of dubious value, lists of titles of lost works, and, in the cases most valuable to modern scholars, extensive transcriptions of original fragments and letters that would otherwise have been lost forever — such as the three complete letters of Epicurus, including a summary of his physics and ethics, or systematic expositions of Stoic logic and Pyrrhonian scepticism. Diogenes Laertius organises the philosophers into two great successive traditions, the 'Ionian' (running from Anaximander to Socrates and the Socratics) and the 'Italian' (from Pythagoras to the Epicureans), a scheme of master-pupil affiliations that, though largely artificial, decisively influenced how the West has traditionally organised the history of ancient philosophy. The work mixes, with little critical discernment, biographical gossip, wordplay attributed to the philosophers, and serious doctrinal material, which has led historians to treat it with caution, yet its importance as a primary source remains irreplaceable precisely because many of the original works it cites have been entirely lost, making Diogenes Laertius the almost sole channel of transmission for a significant portion of ancient Greek thought.