Answer: This seems to be a popular question lately. The answer is kind of lengthy so I'm going to refer you to one of my previous answers. This one seems at first glance not to be the same question, but in the end it works out to the same thing:

-- Question #65

If you still have questions after reading that just get back. It all goes back to the first millennium, and Boethius, Odo of Cluny, Guido of Arezzo, and possibly other people with no surnames.