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We are acquainted with space-time domains which behave

(approximately) in a "Galileian" fashion under suitable choice of

reference-body, i.e. domains in which gravitational fields are

absent. If we now refer such a domain to a reference-body K′

possessing any kind of motion, then relative to K′ there exists a

gravitational field which is variable with respect to space and

time.[17] The character of this field will of course depend on

the motion chosen for K′. According to the general theory of

relativity, the general law of the gravitational field must be

satisfied for all gravitational fields obtainable in this way.

Even though by no means all gravitationial fields can be produced

in this way, yet we may entertain the hope that the general law

of gravitation will be derivable from such gravitational fields

of a special kind. This hope has been realised in the most

beautiful manner.