 describe the
real world, with enough well-defined content referencing objects in the
real world so that it is not merely a set of symbols, then it is not
immediately obvious that it will not be able to represent all reasonable 
theories about the real world.  Mass, time (what we measure on clocks),
and meter are well-defined and provide a grounding for the other 
definitional symbols. Where there are gaps, new concepts can be defined.
If some new concepts are logically incompatible with existing content, 
