Just thinking of people I would consider musical geniuses... beefheart, sun city girls, jandek, john fahey... none of those people had any musical training whatsoever and probably knew jack shit about theory...
But I think most people need a LITTLE bit of theory, in the same way most people need religion. It's good to not go into things completely blind. If you're not going to be a technical virtuoso, at least have plenty of great ideas...
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