We start off again from quite special cases, which we have frequently used before. Let us consider a space time domain in which no gravitational field exists relative to a reference-body _K_ whose state of motion has been suitably chosen. _K_ is then a Galileian reference-body as regards the domain considered, and the results of the special theory of relativity hold relative to _K_. Let us suppose the same domain referred to a second body of reference _K′_, which is rotating uniformly with respect to _K_. In order to fix our ideas, we shall imagine _K′_ to be in the form of a plane circular disc, which rotates uniformly in its own plane about its centre. An observer who is sitting eccentrically on the disc _K′_ is sensible of a force which acts outwards in a radial direction, and which would be interpreted as an effect of inertia (centrifugal force) by an observer who was at rest with respect to the original reference-body _K_. But the observer on the disc may regard his disc as a reference-body which is “at rest”; on the basis of the general principle of relativity he is justified in doing this. The force acting on himself, and in fact on all other bodies which are at rest relative to the disc, he regards as the effect of a gravitational field. Nevertheless, the space-distribution of this gravitational field is of a kind that would not be possible on Newton’s theory of gravitation.[18] But since the observer believes in the general theory of relativity, this does not disturb him; he is quite in the right when he believes that a general law of gravitation can be formulated—a law which not only explains the motion of the stars correctly, but also the field of force experienced by himself. [18] The field disappears at the centre of the disc and increases proportionally to the distance from the centre as we proceed outwards. The observer performs experiments on his circular disc with clocks and measuring-rods. In doing so, it is his intention to arrive at exact definitions for the signification of time- and space-data with reference to the circular disc _K′_, these definitions being based on his observations. What will be his experience in this enterprise?