He never learned the first thing about music

[dar512]

I think it depends on your needs. If you're playing by yourself or in a small group, you can just feel it. However, the larger the group, the more theory you need to understand the music and how to coordinate with the rest of the group. By the time you get to large orchestras, feeling it is a matter of nuance. There's not a lot of room for winging it.

Even in small groups, you might need a mastery of music theory. If you're playing jazz in Chicago and the vocalist or the leader calls for a tune in Ab that is normally played in Bb, then you'd better have a solid theory foundation.

I'm not there ... yet.