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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.