Twenty-six original works of Plato and an extensive range of Aristotle's writings survive; there is also a hymn by the Stoic Cleanthes, and three letters and some maxims from Epicurus, reinforced by the Latin poem of Lucretius. Thales, Pythagoras, Socrates and Pyrrho wrote nothing. For these and the other Greek philosophers in the period BCE we have to rely on quotations and reports from later authors, who refer to their predecessors in different contexts and for widely differing purposes, and vary considerably in accuracy and general reliability. In the case of the Presocratics there is also a network of doxography, that is, comments, summaries and paraphrases of views attributed to the Presocratics on a range of topics. Most of this material is descended, directly or indirectly, from Opinions of the Physicists, a history of early philosophy written by Theophrastus, the student and successor of Aristotle at the Lyceum. In this secondary literature, even with straight quotations it is often difficult to tell where the original words begin and end, and how much of his own terminology and bias the reporting author has introduced. In some cases the quotations are from memory, or reported at second or third hand, where the original work is not available. The problem is further compounded by different and sometimes conflicting manuscript readings.

In addition to the original texts, the following are the main sources available for Greek philosophy, with a note on the authors and their interests.