The Pre-Socratic Greek philosophers were active before Socrates. The popular usage of the term comes from Hermann Diels' work Die Fragmente der Vorsokratiker (The Fragments of the Pre-Socratics, 1903).

Most of what we know about the pre-Socractic philosophers come from quotations by later philosophers and historians. While most of them produced significant texts, none of the texts have survived in complete form.

The standard reference works in English are:

• Gompertz, Theodor 1901. The Greek thinkers: a history of ancient philosophy. Volume 1: the beginnings. London: Murray.
• Guthrie W.K. 1962. A history of Greek philosophy. Volume 1: The earlier presocratics and the Pythagoreans. Cambridge University Press.