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.  To the middle of the lid of the chest is fixed externally a hook with rope attached, and now a being (what kind of being is immaterial to us) begins pulling at this with a constant force. The chest, together with the observer, then begin to move ‘upwards’ with a uniformly accelerated motion.