SOURCE: G. S. Kirk and J. E. Raven, "The Sources for Presocratic Philosophy," in The Presocratic Philosophers: A Critical History, second edition, Cambridge University Press, 1983, pp. 1-6.

Kirk and Raven explain the sources by which the thinking of the Presocratic philosophers have been studied. These sources include: direct quotations of the philosophers found in a variety of sources; testimonia given by Plato, Aristotle, and Theophrastus; and what are known as the doxographies, or collections of philosophical opinions gathered by later scholars.

A. Direct Quotations

The actual fragments of the Presocratic thinkers are preserved as quotations in subsequent ancient authors, from Plato in the fourth century B.C. to Simplicius in the sixth century A.D., and even, in rare cases, to late Byzantine writers like John Tzetzes. The date of the source in which a quotation occurs is not, of course, a reliable guide to its accuracy. Thus...