, which for the present we leave unexplained, occur in it as something essentially new. Now, the extraordinarily fruitful idea that occurred to Einstein was this: Since the new law is to hold for any arbitrary motions whatsoever, thus also for accelerations, such as we perceive in gravitational fields, we must make the gravitation field, in which the observed motion takes place, responsible for the occurrence of these ten factors
. These ten coefficients
which will, in general, be functions of the variables
...
, must, if the new fundamental law is to be of use, be able to be brought into such relationship to the gravitational field, in which the motion takes place, that they are determined by the field, and that the motion described by equation (1) coincides with the observed motion. This is actually possible. (The