Music is, like all, things modeled after the Trinity--a threefold difference within sameness. That's precisely what harmony is--first of all, primarily triadic (as there are only three broad pitch classes that can be simultaneously combined while achieving a truly consonant effect); second of all, it is a coordinated action by separate agents united under one will and purpose; third, a harmony is one thing with many--primarily, three--parts. We simultaneously hear it as one and as more than one. If it were just one, it would not be harmony; and if it were composed of things completely separate, it would not be harmony either. It is, like the Trinity, both at the same time.