Einstein enunciates a general principle that it is possible to find a transformation of coordinate axes which is exactly equivalent to any force, and in particular one which is equivalent to the force of gravitation. That is by concentrating our attention on the transformation which is a purely mathematical operation we can afford to neglect the force completely. To get a better idea of this principle of equivalence as it is called, let us consider a relatively simple example (which actually has nothing to do with gravitation, but which will serve to make our notions clearer.) A person on the earth unconsciously refers all his experiences, i.e., the motions of the objects around him to a set of axes fixed in the earth on which he stands. However, we know that the earth is rotating about its axis, and his axes of reference are also rotating with respect to the space about him. From the point of view of general relativity it is exactly because we do refer motions on the surface of the earth to axes rotating with the earth that we experience the so-called centrifugal force of the earth's rotation, with which everyone is familiar. If we could find it convenient to transform from moving axes to fixed axes, the force would vanish, since it is exactly equivalent to the transformation from one set of axes to the other. However, we find it unnatural to refer daily experiences to axes that are not placed where we happen to be, and so we prefer to take the force and rotating axes instead of no force and fixed axes.]272 We seem to have a direct experience of force in our muscular sensations. By pushing or pulling we can set bodies in motion. It is natural to assume, that something similar occurs, when Nature set bodies in motion. But is this not a relic of animism? The savage and the ancients peopled all the woods and skies with Gods and demons, who carries on the activities of nature by their own bodily efforts. Today we have dispossessed the demons, but the ghost of a muscular pull still holds the planets in place.]141 The general theory is an extension of the special theory which enables the law of gravitation to be deduced. Not in Newton's form, it is true, but in a better form, that is, one that accounts for two important facts otherwise not explained. But it is a far more general theory that indicated above. It is a complete study of the relations between laws expressed by means of any four coordinates (of which three space and one time is a special case), and the same laws expressed in the four coordinates of a system having any motion whatever with respect to the first system. By restricting this general study in accordance with certain postulates about the nature of the universe we live in, we arrive at a number of conclusions which fit more closely with observed facts that the conclusions drawn from Newton's theory.]221 NOTES