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If we desire to adhere to our "old-time" three-dimensional view

of things, then we can characterise the development which is

being undergone by the fundamental idea of the general theory of

relativity as follows: The special theory of relativity has

reference to Galileian domains, i.e. to those in which no

gravitational field exists. In this connection a Galileian

reference-body serves as body of reference, i.e. a rigid body the

state of motion of which is so chosen that the Galileian law of

the uniform rectilinear motion of "isolated" material points

holds relatively to it.

Certain considerations suggest that we should refer the same

Galileian domains to non-Galileian reference-bodies also. A

gravitational field of a special kind is then present with

respect to these bodies (cf. Sections XX and XXIII).