Many of the bands I used to play in included great players, not just enthusiasts, but musicians who had a great talent, feel, and brought something new to the table with their performance on their chosen instrument.
However if you asked them to notate something they'd written, or in some cases even tell you what the root note of one of the chords they were playing was, you'd see a blank face in return.
In my own experience, formal music training was a real foundation for my ability to "know my way around" the keyboard, anticipate changes, and to help understand why other musicians did what they did. Scales, reading music, understanding the value of silence, dynamics, light and shade, etc. didn't tell me what to play - but it gave me a language in which to speak and interpret music.
My years of formal training were not the most enjoyable - possibly due to my youth and wanting to do my own thing at the time - but they were the first steps towards being able to talk the walk and add an additional depth to my thoughts in composition, either solo or band-based, in following years.
One of the most original, musicially sensitive and just plain enjoyable guys to play with that I know had no musical training at all. I've never heard him sing a duff note, he has an amazing sense for harmony and capturing emotion through the combination of lyric and guitar chords, and has a great ability to work with the other musicians around him in building a song right up to its peak, or letting it settle right back in a soft, subtle groove when it just felt right.
But ask him what the note was he was playing in some elaborate chord and you'd spend a few minutes hunting for the odd yet strangely working combination of notes he'd stumbled onto.
Sometimes I'd wonder whether his lack of formal musical training gave him a greater sense of creative freedom, as though the "rules" or typical structures he may learn through formal training would box him in or restrict where he would have otherwise taken things.
In my friend's case he has always been a dedicated fan of the music of Queen, The Beatles, Cat Stephens, and others. When surrounding your ears with that musical company, I don't think you could creatively pick up a guitar and set foot in the wrong direction - with or without a lesson.
I remember reading that the acclaimed film composer Hans Zimmer, responsible for some of the most memorable scores in history (Rain Man, The Lion King, Pirates of the Caribbean...) disliked and discontinued piano lessons at a young age as he didn't enjoy the discipline.
In the end I believe we are all products of (and arguably victims of) our creative inputs. In my opinion, with the right creative inputs, great music composition can be achieved even without a background in formal music training. But it sure helps in understanding what you're doing and communicating it with others!
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