The diverse responses to the first posting on this subject make it clear that it was unnecessarily obscure.
It was an attempt to deal metaphorically with the fundamental question, “What makes some music good and other music not-so-good?” Its primary message was the seemingly preposterous proposition that the quality of a melody can be assessed by the extent to which it seems to have existed before it was composed. Trying to prove such a proposition empirically would be a waste of time, like trying to demonstrate that the Beatles meant it literally when they sang:
Roll over Beethoven
Roll over Beethoven
Roll over Beethoven
Roll over Beethoven
Roll over Beethoven
And dig these rhythm and blues
Or that Beethoven in his Ninth Symphony meant it literally when he set to music Schiller’s demand, One Kiss to the Whole World!
The metaphor comes out of the Italian Renaissance. When Michelangelo began chiseling away at the marble he had chosen in the Carrara quarries to create Moses or David or the Pietà, or whatever, he proceeded to work away at it until he had uncovered the figure that was already there. All he had to do was to lay it bare. According to this legend, which was perhaps planted by Michelangelo himself, he did not create his art – he discovered it. And thus, by analogy, many superb melodies were waiting to be discovered by the Beatles, by Beethoven and by Wolfgang Amadeus Mozart. Père Leopold had to chisel away for weeks only to discover a few scattered pebbles.
The metaphor of the artist as discoverer seems to convey the important truth that in order to make an unheard melody heard it had to be discovered. Is it not impossible to believe that Schubert actually invented the melody of The Trout? Is it not much more plausible that he found it somewhere?
A respondent wrote: W.A. Mozart and F.J. Haydn succeed as composers in ways that their relatives L .Mozart and M.Haydn don’t.… What makes Mozart uniquely Mozart and Haydn uniquely Haydn?
The answer: the best pre-existing melodies W.A. Mozart and F.J. Haydn discovered fell into their laps and they knew what to do with them. Compared to them, their poor relatives had to slave away for unimpressive, yet respectable results.
The personal stamp that composers and artists impose on their works is as unique as their fingerprints. Unlike the quality of their art, which is a matter of aesthetic judgment, it can be described and analyzed.
But not today.
Eric Koch’s book, The Weimar Triangle, is available at Indigo-Chapters and in your local bookstore. 
It’s a good answer, but not to the question I think your respondent meant to ask, namely: What makes Haydn Haydn and not Mozart? What makes Mozart Mozart and not Haydn? How come they heard _different_ pre-existing melodies??
But still unanswered is the question what is good music and what is not-so-good music. Who decides?
My respondent is asking for too much.
I don’t know.
My respondents rarely ask questions to which they don’t know the answer themselves.
This is a rare instance, then.
I too have no good answer.
_Of course_ I ask for too much! What’s the point of asking for too little, or just barely enough? Have I not learnt at your generous knee that good questions have no easy answers?
It’s very subjective. I like Mahler but my uncle, a professional musician-conductor, thought Bruckner was better. As a rule you like familiar music, music you recognize. But basically, I do not have an answer as to quality. I do not believe there are objective standards.
Eric,
Please give Chuck Berry his due!
He is the composer/lyricist of “Roll Over Beethoven.”
He was gratefully resentful of the Beatles for popularizing his song to the teeming masses.
–Alan
May I welcome you to the distinguished group of
respondents.
Chuck Berry’s time (in my blog) will come.
Chuck Berry remains a saint until this day, and “THE King of Rock & Roll,” in spite of Jerry Lee Lewis.
Perhaps Eric prefers Liona Boyd?
Harry Armstrong
Yes, I do – especially since lunch today when my memory of Liona was so agreeably refreshed.
Unlike Trudeau, I have never met her.
But I HAVE met her father.