Consider any material system whatever. We have to consider on the one hand the "state" of the various bodies of this system--for example, their temperature, their electric potential, etc.; and on the other hand their position in space. And among the data which enable us to define this position we distinguish the mutual distances of these bodies that define their relative positions ... [p. 76] The law of the phenomena which will be produced in this system will depend on the state of these bodies, and on their mutual distances; ... they will not depend on the absolute position and orientation of the system. ... This is what we shall call, for the sake of abbreviation, the law of relativity.
Published before 1923