Okay, I understand that (I think).
So, what's outside the Earth and the bits and bobs that come with us? Acceleration is defined as the rate of change of velocity, correct? And velocity can be defined as the rate of change of displacement, again, correct? And displacement is the direct, straight line distance from a point, so there has to be some fixed point from which acceleration can be garnered. What is this place? It can't be nothing, or an arbitrary point, as that nullifies the whole acceleration argument.