we may assume with him that there is not a single physical or
chemical phenomenon--which does not feel, although very probably in
an unnoticeable degree, the influence of gravitation, and that, on the
other side, the attraction exercised by a body is limited in the first
place by the quantity of matter it contains and also, to some degree,
by motion and by the physical and chemical condition in which it moves.

It is comprehensible that a person could not have arrived at such a
far-reaching change of view by continuing to follow the old beaten
paths, but only by introducing some sort of new idea. Indeed,
Einstein arrived at his theory through a train of thought of great
originality. Let me try to restate it in concise terms.



THE EARTH AS A MOVING CAR

Everyone knows that a person may be sitting in any kind of a vehicle
without noticing its progress, so long as the movement does not vary
in direction or speed; in a car of a fast express train objects fall
in just the same way as in a coach that is standing still. Only when
we look at objects outside the train, or when the air can enter the
car, do we notice indications of the motion. We may compare the earth
with such a moving vehicle, which in its course around the sun has
a remarkable speed, of which the direction and velocity during a
considerable period of time may be regarded as constant. In place
of the air now comes, so it was reasoned formerly, the ether which
fills the spaces of the universe and is the carrier of light and of
electro-magnetic phenomena; there were good reasons to assume that the
earth was entirely permeable for the ether and could travel through it