Think of waves on the surface of water. Here we can describe two
entirely different things. Either we may observe how the undulatory
surface forming the boundary between water and air alters in the course
of time; or else--with the help of small floats, for instance--we
can observe how the position of the separate particles of water
alters in the course of time. If the existence of such floats for
tracking the motion of the particles of a fluid were a fundamental
impossibility in physics--if, in fact, nothing else whatever were
observable than the shape of the space occupied by the water as it
varies in time, we should have no ground for the assumption that
water consists of movable particles. But all the same we could
characterise it as a medium.

We have something like this in the electromagnetic field. For we may
picture the field to ourselves as consisting of lines of force. If
we wish to interpret these lines of force to ourselves as something
material in the ordinary sense, we are tempted to interpret the
dynamic processes as motions of these lines of force, such that each
separate line of force is tracked through the course of time. It is
well known, however, that this way of regarding the electromagnetic
field leads to contradictions.

Generalising we must say this:--There may be supposed to be extended
physical objects to which the idea of motion cannot be applied.
They may not be thought of as consisting of particles which allow
themselves to be separately tracked through time. In Minkowski's
idiom this is expressed as follows:--Not every extended conformation
in the four-dimensional world can be regarded as composed
of world-threads. The special theory of relativity forbids us to
assume the ether to consist of particles observable through time,