depends on the motion of the observer relative to the system. There is no criterion which may be applied to a set of descriptions, by means of which a single true description may be determined. All the descriptions are true. The reason is that if we carry our analysis far enough, we are bound to consider the fact as a whole, namely, not only is there an object, but the object is seen by an observer. Consequently each perception is a different fact, and even admitting the object to be the same, for the purposes of argument, the descriptions, though all true, will be different, for each actually involves the observer and the observation as well as the object observed. Hence physics, which purports to describe things independent of any particular perceiving subject, is compelled in the end to take account of that subject. This is inevitable, seeing that the concepts of physics are constructions based in the first place on individual perceptions. In fact, the principle of relativity, as applied to physical science, is a particular example of the more general philosophic fact that while the experience of the subject is objective and absolute for him, it is subjective and relative from the universal conceptual stand- point.
In considering existence, then, from the conceptual point of view, we are continually brought face to face with its relativity. This is the root of the difficulty in the problem of change. As regards physical science, Minkowski succeeded in transcending the difficulty of relativity by introducing this idea of a space- time world. In this way he not only made clear the source of the trouble, but also indicated how it might be eliminated in analysis. It is simply a question of taking a wider view of existence; and in considering an individual who changes and yet maintains his identity, we shall get rid of the difficulties to a great extent if we proceed on similar lines. In specifying an individual, reference must be considered to be made to a space-time unity. In such a proposition as 'A went to London on Saturday,' A must not be supposed to be specified by any time or place. A is a space-time entity whose existence is considered as a whole. The proposition, though it contains a spatial and temporal reference, is asserted of this individual whole, which transcends both space and time.
