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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.