Reference: Einstein’s 1920 Book
Section III (Part 1)
Space and Time in Classical Mechanics
Please see Section III at the link above.
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Summary
“Space,” most likely, means a volume free of matter for a material object to exist in. The “position” of an object in that space can only be specified by referencing it from another object using the coordinate system. Thus, we always see the trajectory of a body relative to another body, and not independently.
Time is measured by the regular ticks of a clock that are standardly spaced apart. This clock is attached to the reference body. The motion of the object is determined by noting its position at every tick of the clock. The time coordinate should also take into account the time light takes in traveling from the object to the origin point on the reference body.
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Comments
Einstein notes that the space coordinates shall be affected by the degree of rigidity of the coordinate system, and the time coordinates shall be affected by the finiteness of the velocity of light.
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