The purpose of this book is to give, through a diligent selection of Einstein's writings, a concise, but clear and unified representation of his scientific doctrines and philosophical conceptions. His writings are not many: short memoirs, a few fairly succinct essays, lectures and interviews, are all that can be drawn upon to form a concrete picture of his philosophy and his attitude toward life. In order to allow the non-specialized reader to understand the essence and value of the theories of relativity, we have reproduced in their original text some of Einstein's writings and speeches intended for an audience of non-mathematicians and which briefly expose the genesis and character of those theories. Written in a concise and clear language, without references, with rare exceptions, to symbols and formulas of high mathematics, they will give to every reader of average preparation a clear vision of Einstein's theories and of the logical evolution of the thought that conceived and conducted them.