Einstein emphasized the importance of general covariance for the development of general relativity, and took the position that the general covariance of his 1915 theory of gravity ensured implementation of a generalized relativity principle. This view was challenged by Erich Kretschmann (1917), who argued that every theory of space and time (even including Newtonian dynamics) can be formulated in a covariant way, if additional parameters are included, and thus general covariance of a theory would in itself be insufficient to implement a generalized relativity principle. Although Einstein (1918) agreed with that argument, he also countered that Newtonian mechanics in general covariant form would be too complicated for practical uses. Although it is now understood that Einstein's response to Kretschmann was mistaken (subsequent papers showed that such a theory would still be usable), another argument can be made in favor of general covariance: it is a natural way to express the equivalence principle, i.e., the equivalence in the description of a free-falling observer and an observer at rest, and thus it is more convenient to use general covariance together with general relativity, rather than with Newtonian mechanics. Connected with this, also the question of absolute motion was dealt with. Einstein argued that the general covariance of his theory of gravity supports Mach's principle, which would eliminate any "absolute motion" within general relativity. However, as pointed out by Willem de Sitter in 1916, Mach's principle is not completely fulfilled in general relativity because there exist matter-free solutions of the field equations. This means that the "inertio-gravitational field", which describes both gravity and inertia, can exist in the absence of gravitating matter. However, as pointed out by Einstein, there is one fundamental difference between this concept and absolute space of Newton: the inertio-gravitational field of general relativity is determined by matter, thus it is not absolute.