Even if you don't know theory from a book, anyone who is really an accomplished song writer knows theory. They may not know what the scale is called, but they know how to come up with things that have a certain sound. As you progress as a musicial you basicly teach yourself theory without any names or terms, but once you know your shit you know how to get certain sounds just from experimentation over years. But obviously its alot easier if you know the stuff beforehand.

__________________