He never learned the first thing about music

[limey]

I think that a lot of the point of learning music theory is to enable you to communicate with other musicians - whether it's to write down a tune and harmonisation to share with people who are not present, or to identify where to go back to in a rehearsal. Being able to create a great melody and devise harmony for it (on a guitar, say) doesn't necessarily require knowledge of the theory behind it, IMHO; just as knowing all the theory on earth doesn't necessarily mean you'll write great tunes.