As blind people might do, each in turn reaches out and touches the elephant lightly, scanning and feeling with his fingers. Each blind man had in fact described quite accurately the section of the elephant he had examined. The problem was that none of them could really get the whole picture . Well, it's the same with music theory. Let me try for a descriptive definition.
Music theory is a collection of harmonic principles which form the basis of our musical heritage .
We could safely say that music theory is our elephant, and all of us guitar teachers are blind men. We may each have our own different way of presenting this knowledge, but it's all basically the same knowledge, just presented from a different viewpoint. There is no one ultimate right way to describe the whole body of knowledge any more than I could show you a circle and say this is where it starts. Both music theory and a circle are continuous and all connect together. I'm telling you this because I want you to understand why you see so many different approaches to it.