"All [of Newton's] fundamental laws of mechanics involved statements concerning accelerations, changes in the velocities... rather than the velocities themselves. These accelerations were tied to the distances between the bodies... [F]or collecting data relevant to an experimental confirmation of Newton's laws... one may consider equivalent all observers who, relative to one another, are engaged in straight-line and unaccelerated motion. ...Such an observer will be called an inertial observer; relative to him, the motion of a forcefree body will be unaccelerated. If an inertial observer is considered the hub of a scaffolding... one calls the whole framework an inertial frame of reference, or for short, an inertial frame. ...The equal validity of all inertial frames... and the non-existence of one frame representing absolute rest, is known as the principle of relativity. [It] remained unquestioned for about two hundred years. ...[T]here was no such thing as absolute rest, or absolute motion, for that matter, but only absolute acceleration... governed by the forces resulting from the proximity of other bodies."