Although each of the essays in this volume is a study complete in itself, they are connected by one and the same synthetic spirit, and are animated by one and the same object: that of demonstrating the utility in the biological, psychological, and sociological fields of the theorist, who, without having specialized in any particular branch or subdivision of science, may nevertheless bring into those spheres that synthetic and unifying vision which is brought by the theorist mathematician, with so much success, into the physico-chemical field of science.