My text book basically defines an inertial reference frame as follows: If you have an object O that has no forces acting on it, and there is a reference frame R where the acceleration of O with respect to R is zero, then R is a inertial reference frame. This to me seems circular. How does one know if there are no forces acting on O? Don't you need to know if O has any acceleration? How do you know that O has no acceleration? Don't you need an inertial reference frame? Stated differently: The definition stated above seems, to my understanding, to say that an inertial reference frame needs to be a reference frame that does not accelerate. And basically I am asking how do you know if it is accelerating? Can you really know with out already having an inertial reference frame? Thanks.