The anthyphairetic reconstruction of the original Pythagorean proof of incommensurability, by means of the restoration of Book II of the Elements to its original Pythagorean form

Abstract: Unquestionably the greatest discovery of the Pythagoreans is the existence of incommensurable magnitudes, most probably the incommensurability of the diameter to the side of a square, but there is no agreement among historians of Greek mathematics on their method of proof. In this chapter we present novel arguments not only for an anthyphairetic reconstruction of the original Pythagorean proof of incommensurability, but also in favor of one that employs the Pythagorean Application of Areas in Excess and in fact Geometric Algebra. The main tool for this reconstruction is the restoration of Book II of the Elements to its original Pythagorean form. The final version of this paper will appear as a chapter in the book Essays on Geometry: Celebrating the 65th Birthday of Athanase Papadopoulos, ed. A. Muhammed Uluda\u{g} and A. Zeytin, Springer International Publishing, 2025.