preserves its form after any arbitrary transformation of the variables, all systems of reference are equally justified as such, i.e. there is none which is more important than any other.
The most important part of the problem, with which Einstein saw himself confronted, was the setting-up of differential equations for the gravitational potentials of the new theory. With the help of these differential equations, the
's were to be unambiguously calculated (i.e. as single-valued functions) from the distribution of the quantities exciting the gravitational field; and the motion (e.g. of the planets) which was described, according to equation (1) by inserting these values for the
's, had to agree with the observed motion, if the theory was to hold true. In setting up these differential equations for the gravitational potentials
Einstein made use of hints gathered from Newton's theory, in which the factor which excites the field in Poisson's equation
for the Newtonian gravitational potential (viz. the factor represented by