"The elucidations which I here offer are, in part, substantially contained in my treatise, Die Geschichte und die Wurzel des Satzcs von der Erhaltung der Arbeit (...1872). At a later date nearly the same views were expressed by Kirchhoff (Vorlesungen über mathematische Physik: Mechanik, Leipsic, 1874) and by Helmholtz (Die Thatsachen in der Wahrnehmung, Berlin, 1879), and have since become commonplace enough. Still the matter, as I conceive it, does not seem to have been exhausted, and I cannot deem my exposition to be at all superfluous."