I've been told by many people that music theory is not normative; it doesn't tell you what to write or do in music to create something beautiful that sounds good. I've been told that it's strictly descriptive; that is, it describes a set of observed patterns in music that (someone) thinks sounds good, or that is frequently written.
So with that in mind, I want to ask about theory and composition. How is theory supposed to inform composition? Some musicians seem to have a deep theoretical knowledge, and regardless of what they would say their mental process is, this seems to affect the way that they compose, since they can't "un-learn" certain patterns. On the other hand, there are untutored musicians who know nothing about theory, and who in some sense are more "free to break the rules", being unaware of what the rules are.
EDIT - This question has already gotten a lot of attention, and several good answers. I'm starting to think the original question is a bit flawed; here are three big things I've gleaned:
- Several answers point out that people who don't know any formal theory still adhere to many patterns described by formal theory, in the way that a child who speaks English knows sentence grammar without knowing the terms of academic structure of grammar. In this way perhaps, they're "using theory" without having a conscious awareness of rules and terminology like chromatic mediants, modulation, etc.
- Another perspective here is that focusing on the connection between theory and composition is the wrong focus, because what matters is whether the result is good, pleasing, unexpected (pick your descriptive term). If it is, then it doesn't matter what the theory connection was. If it isn't, then clearly it also doesn't matter what the theory connection was.
- There's a fair amount of concern about the interplay between theory and creativity. Does knowing theory make you more creative, or less creative? There's no consensus, it appears the answer is "both, depending on who you talk to".