An analysis of Euclid's geometrical foundations

Abstract

The initial techniques developed in Euclid's Elements, well before the use of the parallel postulate, are reexamined in order to clarify even the most obscure details, particularly those related to equality, superposition and angle comparison. Some commentary on modern developments is included. The known but often misunderstood implicit handling of betweenness and points of intersection is briefly treated. We also sketch a rigorous treatment of absolute geometry in a spirit similar to Euclid's, one that allows properties of angles and triangles to be derived from two simple axioms on right angles, which then leads to rigid motions of certain planar geometries.