We advance a step farther in our generalisation when we express the tenet thus: If, relative to _K_, _K′_ is a uniformly moving co-ordinate system devoid of rotation, then natural phenomena run their course with respect to _K′_ according to exactly the same general laws as with respect to _K_. This statement is called the _principle of relativity_ (in the restricted sense). As long as one was convinced that all natural phenomena were capable of representation with the help of classical mechanics, there was no need to doubt the validity of this principle of relativity. But in view of the more recent development of electrodynamics and optics it became more and more evident that classical mechanics affords an insufficient foundation for the physical description of all natural phenomena. At this juncture the question of the validity of the principle of relativity became ripe for discussion, and it did not appear impossible that the answer to this question might be in the negative. Nevertheless, there are two general facts which at the outset speak very much in favour of the validity of the principle of relativity. Even though classical mechanics does not supply us with a sufficiently broad basis for the theoretical presentation of all physical phenomena, still we must grant it a considerable measure of “truth,” since it supplies us with the actual motions of the heavenly bodies with a delicacy of detail little short of wonderful. The principle of relativity must therefore apply with great accuracy in the domain of _mechanics_. But that a principle of such broad generality should hold with such exactness in one domain of phenomena, and yet should be invalid for another, is _a priori_ not very probable.