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The older I get, the more I'm attuned to the limitations of talent like Metheny. It's like how there are great mathematicians, and many of them can be strong problem-solvers and do great work, but can still fall short of "genius", because - and this is my hypothesis - they're too strong at tackling fine-grained details immediately, so they don't actively seek out the kind of abstractions that would lead to a different perspective. It's like trying to explain how you walk: "it's obvious." (even though at some point you did struggle with it) When I hear synthestites talk, they are trying to explain how walking works - it's tapping into neural pathways that are wired into the subconscious, skipping over any preliminary decoding. So they often create things that sound marvelous, bring in tons of techniques, but are at their core heavily improvised with minimal "concept" - elequent baby babbles.

For the rest of us creativity is achieved by adapting between different symbolic contexts, and this helps us explain our results when we get them, and highlights using structural abstractions. So for example music with a heavy lyrical component usually isn't in the domain of the synthestetic prodigy, because it needs crossover between poetic/storytelling skills and musical ones. They can do it, but not with the same fluidity with which they can just "sit at the keys" and get swept away.

All that said, I think Metheny's right about melody. There are tricks to improve what a melody communicates, but no particular formula can benchmark whether or not it works in the way that you can benchmark playing inside a rhythm, scale or harmony.