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Every description of the scene of an event or of the position of

an object in space is based on the specification of the point on

a rigid body (body of reference) with which that event or object

coincides. This applies not only to scientific description, but

also to everyday life. If I analyse the place specification

"Trafalgar Square, London"[3] I arrive at the following result.

The earth is the rigid body to which the specification of place

refers; "Trafalgar Square, London" is a well-defined point, to

which a name has been assigned, and with which the event

coincides in space.[4]

[3]

I have chosen this as being more familiar to the English reader

than the "Potzdammer Platz, Berlin," which is referred to in the

original. (R. W. L.)

[4] It is not necessary here to investigate further the

significance of the expression "coincidence in space." This

conception is sufficiently obvious to ensure that differences of

opinion are scarcely likely to arise as to its applicability in

practice.