the motion of the raven would be one of different velocity and
direction, but that it would still be uniform and in a straight line.
Expressed in an abstract manner we may say: If a mass _m_ is moving
uniformly in a straight line with respect to a co-ordinate system _K_,
then it will also be moving uniformly and in a straight line relative
to a second co-ordinate system _K _ provided that the latter is
executing a uniform translatory motion with respect to _K_. In
accordance with the discussion contained in the preceding section, it
follows that:
If _K_ is a Galileian co-ordinate system. then every other co-ordinate
system _K _ is a Galileian one, when, in relation to _K_, it is in a
condition of uniform motion of translation. Relative to _K _ the
mechanical laws of Galilei-Newton hold good exactly as they do with
respect to _K_.
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
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