Diogenes Laertius c. 180 – c. 240

Diogenes Laertius was a Greek biographer of philosophers of the third century AD, the author of the Lives and Opinions of Eminent Philosophers, the most extensive surviving ancient compilation of philosophical biography and doxography. The ten books treat in succession the schools of Greek philosophy from the seven sages and Thales through Plato, Aristotle, the Cynics, the Stoics, the Skeptics, and the Epicureans, ending with a substantial extract of Epicurus's letters. Although his judgments are often eccentric and his quotations not always accurate, his work is, for many of the philosophers he treats, the only source we have, and has shaped the historiography of ancient philosophy for nearly two millennia.

Diogenes Laertius is the otherwise almost unknown author of the Lives and Opinions of Eminent Philosophers, the single most important biographical and doxographical source for ancient Greek philosophy. From scattered internal evidence he is conjectured to have lived in the early third century AD, perhaps in Cilicia, and to have completed his work under or shortly before the Severan dynasty. Apart from his book nothing reliably survives of him: he names no city, no school, and almost no personal commitments.

The work is a ten-book history of philosophy organised by school and lineage, beginning with the Seven Sages and Thales, running through the Ionian, Italian, Socratic, and Megaric traditions, and ending with the Pyrrhonist sceptics and the Epicureans. It preserves biographical anecdotes, lists of works, fragments, verbatim letters, and entire technical summaries of doctrine — most famously the three letters and the Principal Doctrines of Epicurus that we have only because Laertius copied them.

His sympathies are difficult to fix: he is more credulous than the great philosophers he writes about, structurally hospitable to the sceptics, and warmly appreciative of the Epicureans, but never identifies himself with any school. The Lives is uneven in quality and often legendary in detail, but without it the history of Greek philosophy as we have it would be unrecognisable. He is presumed to have died around AD 240.