The article deals with physical relativity and with philosophical relativism against the background of the emergence and formulation of Einstein's theory of relativity. The question is posed as to whether either ontological or cosmological relativism is defensible on the basis of a treatment of the existence of possible alternative theories of reality: that is, relative, equivalent and non-equivalent theories. The discussion of the possible relativity of theory leads to the following conclusions: (1) the fundamental part of one theory may be a part of some other theory, (2) that the existence of alternative relative theories, though it may be the result of methods and approaches to reality, may only be consistent and relevant in result if that result is couched in some mathematically equivalent theory, (3) that prediction of equivalent and relative theories shows a different of stability in the testing of their deductive consequences, and that (4) for the further development and success of the equivalent theory the role of their abstract, universal and unifying principles is decisive. These conclusions together with Einstein's assumption of the existence of universal and elementary laws from which the cosmos might be inferred by pure deduction, and strengthening tendencies towards the uncovering of deeper and more universal connections in physics, all lead to the thought that ontological relativism is in this sense untenable.
