Mach's Principle is regarded as a forerunner of Einstein's Theory of Relativity, but is it a legitimate principle in its own right? http://en.wikipedia.org/wiki/Mach's_principle Does Mach's Principle imply an origin of inertia? - ie. does it mean that a body can only experience inertia if there are other bodies in the universe to pull on it or interact with it? What happens if a body is just sitting alone in the universe, and it starts rotating? Will it feel any rotational inertia in the absence of other bodies in the universe? So does inertia originate from a body's interaction with other masses in our universe, or does it originate from a body's interaction with space itself?