Diogenes Laërtius (/daɪˈɒdʒᵻniːz leɪˈɜːrʃiəs/; Greek: Διογένης Λαέρτιος, Diogenēs Laertios; fl. c. 3rd century AD) was a biographer of the Greek philosophers. Nothing is definitively known about his life, but his surviving Lives and Opinions of Eminent Philosophers is a principal source for the history of Greek philosophy.

Writings

-Lives and Opinions of Eminent Philosophers

The work by which he is known, Lives and Opinions of Eminent Philosophers, was written in Greek and professes to give an account of the lives and sayings of the Greek philosophers. Although it is at best an uncritical and unphilosophical compilation, its value, as giving us an insight into the private lives of the Greek sages, led Montaigne to exclaim that he wished that instead of one Laërtius there had been a dozen. On the other hand, modern scholars advise that we treat Diogenes' testimonia with care, especially when he fails to cite his sources: "Diogenes has acquired an importance out of all proportion to his merits because the loss of many primary sources and of the earlier secondary compilations has accidentally left him the chief continuous source for the history of Greek philosophy." Werner Jaeger damned him as "that great ignoramus".
It has been suggested that Diogenes was an Epicurean or a Skeptic. He passionately defends Epicurus in Book 10, which is of high quality and contains three long letters (attributed to Epicurus) explaining Epicurean doctrines. He is impartial to all schools (in the manner of the ancient skeptics), and he carries the succession of Pyrrhonism further than the other schools. At one point, he even seems to refer to the Skeptics as "our school." On the other hand, most of these points can be explained by the way he uncritically copies from his sources. It is by no means certain that he adhered to any school, and he is usually more attentive to biographical details.