Now it was starting to get interesting, discussing the "rules" of music, as applied to music. Now if you analyse a nbit of music, you soon spot some patterns emerging. You might see that pieces by Mozart virtually never use parallel 5ths; or you might spot that a certain fiddler's tunes were all in the "modes"( in the modern, classification by scale-type sense:Ionian, Myxolydian etc). Now is the "no parallel 5ths" pattern a "rule" that Mozart was following, or is it a description of Mozart's composing style? Or, possibly, something between the two? And that fiddler: is he/she consciously adjusting the repertoire so all the tunes are consistently "modal", or is he just playing the tunes that he likes(or remembers).
   This is fairly crucial to any understanding of musical "theory". same with grammar. We mostly have a good, consistent way of using definite and indefintie articles (whether we say "a tree" or "the tree" according to context). But how many people have ever given this much though, or could beging to explain rationally the "rules" that determine when either one is used. The fact is, for most people it is instinctive. You dont need to "know the rues" to get it right. It's a different bit of your brain.
   Same with modes.