This is an updated edition of a groundbreaking examination of early Greek mathematics. The author has revised parts of the text, updated the bibliography, and added a new Appendix where he takes a strong position in the continuing debate about the nature and range of classical mathematics. The first part presents several new interpretations of the idea of ratio in early Greek mathematics and illustrates these in detailed discussions of several texts. Part Two then focuses on the sources themselves and provides a critical look at our knowledge of Plato's Academy during his lifetime, at the source of our text of Euclid's Elements, and at our understanding of early Greek mathematics. The final part contrasts some of the evidence from early and late antiquity and then gives a historical account, beginning in the seventeenth century, of the modern theory of continued fractions, which underlies our reconstruction of early Greek mathematics.