Sixteen essays trace seminal ideas about the foundations of geometry that led to the development of Einstein's general theory of relativity. This original Dover publications is the only English-language collection of the important papers, some of which are extremely hard to find. Contents include: Bernhard Riemann, "On the Hypotheses That Lie at the Foundations of Geometry". Hermann von Helmholtz, "On the Factual Foundations of Geometry". William Kingdon Clifford, "On the Space-Theory of Matter". Henri Poincaré, "Non-Euclidean Geometries". Felix Klein, "The Most Recent Researches in Non-Euclidean Geometry". Élie Cartan, "Euclidean Geometry and Riemannian Geometry". Albert Einstein, "The Problem of Space, Ether, and the Field in Physics". These remarkably accessible papers will appeal to students of modern physics and mathematics, as well as anyone interested in the origins and sources of Einstein's most profound work. Peter Pesic of St. John's College in Santa Fe, New Mexico, provides an introduction, as well as notes that offer insights into each paper.