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We imagine a large portion of empty space, so far removed from

stars and other appreciable masses, that we have before us

approximately the conditions required by the fundamental law of

Galilei. It is then possible to choose a Galileian reference-body

for this part of space (world), relative to which points at rest

remain at rest and points in motion continue permanently in

uniform rectilinear motion. As reference-body let us imagine a

spacious chest resembling a room with an observer inside who is

equipped with apparatus. Gravitation naturally does not exist for

this observer. He must fasten himself with strings to the floor,

otherwise the slightest impact against the floor will cause him

to rise slowly towards the ceiling of the room.