It's hard for me NOT TO see Propositional Logic (or other
   systems using symbols as such) as equivalent to programming
   languages. Programming language are entirely metaphorical in
   nature; unbelievably so.

   When you call a function name? it's a precise metaphor for the
   function that works within it.

   HOWEVER: Why is it a metaphor and not equal?

   Because variables can global or local. They can be destroyed.
   Meanings can change. It's only within a limited context that
   they are valid.

   They're metaphorical because they don't mean the same thing in
   every context. If I use P in Propositional Calculus, I must use
   a specific Glossary of meanings (meanings are metaphorical as
   they pull in external knowledge from other sources to help shape
   the sense that this particular P is desired to be used in....
   similarly to how a politician will shape meaning by making
   metaphorical comparisons to "The Old West" when describing the
   fight against our enemies (whatever they are at the time).

   The metaphors shape the sense in which the meaning is inferred
   within the minds of the audience.

   If the audience is a bunch of students of Logic, well, there's
   your politician and your constituents.

   In both cases, a pragmatic/practical aim is desired: Thinking
   clearly about "something".

   Whether or not that mode of thinking is *desirable* depends on
   the politics of the situation. What's the desired intention of
   the speaker/teacher/educator? It takes many metaphors /
   analogies to shape a desired sense in the mind of the student.

   Much the same way, it takes much coaxing to get electromagnetic
   devices to conform to the intentions of the hardware designer to
   make a computer do what a computer does.

   I could go deeper, but then I will appear entirely loony, if not
   already tongue emoticon