Among the conditions which have made discussion of relativity and ether difficult is that not enough attention has been given to the use of very different meanings of the words "relative" and "relativity" in physics. I intend in what follows to distinguish two concepts of relativity from each other by the designations "relativity of bodies" [Körperrelativität] and "relativity of representation" [Darstellungsrelativität]. - It was Ernst Mach who brought forth the idea of the relativity of bodies and who directed it against Newton's absolute theory of space and time. When one says with Newton that unattached bodies move uniformly in a straight line, Mach rejected the idea that therefore space and time must exist in themselves and become important as something given in the universe. According to Ernst Mach (Die Mechanik in ihrer Entwicklung, Leipzig, 1. Auflage 1883, 7. 1912), absolute space is a "desperate" [verzweifelter] thought: "If we therefore say that a body maintains its direction and velocity in space, that is merely an abbreviated way of saying with respect to the whole world."