permanent world. But the permanent world so found demands the partitioning of space-time in one of a certain number of ways, viz. those discussed in Chapter iii[1]; from these a particular space and time are selected, because the observer wishes to consider himself, or some arbitrary body, at rest. This gives the space and time used for ordinary descriptions of experience. In this way we are able to introduce perceptual space and time into the four-dimensional world, as derived concepts depending on our desire that the new-found matter should be permanent.
I think it is now possible to discern something of the reason why the world must of necessity be as we have described it. When the eye surveys the tossing waters of the ocean, the eddying particles of water leave little impression; it is the waves that strike the attention, because they have a certain degree of permanence. The motion particularly noticed is the motion of the wave-form, which is not a motion of the water at all. So the mind surveying the world of point-events looks for the permanent things. The simpler relations, the intervals and potentials, are transient, and are not the stuff out of which mind can build a habitation for itself. But the thing that has been identified with matter is permanent, and because of its permanence it must be for mind the substance of the world. Practically no other choice was possible.
It must be recognised that the conservation of mass is not exactly equivalent to the permanence of matter. If a loaf of bread suddenly transforms into a cabbage, our surprise is not diminished by the fact that there may have been no change of weight. It is not very easy to define this extra element of permanence required, because we accept as quite natural apparently similar transformations—an egg into an omelette, or radium into lead. But at least it seems clear that some degree of permanence of one quality, mass, would be the primary property looked for in matter, and this gives sufficient reason for the particular choice.
We see now that the choice of a permanent substance for the
- ↑ When the kind of space-time is such that a strict partition of this kind is impossible, strict conservation does not exist; but we retain the principle as formally satisfied by attributing energy and momentum to the gravitational field.
