  The problem is a familiar one; how to introduce a class of preferred motions, that is, how to characterize that particular path structure that would govern the motions of free particles \(\mathcal{P}_{f}\), that is, neutral, spherically symmetric, non-rotating test bodies, while avoiding the circularity problem surrounding the notion of a free particle: The only way of knowing when no forces act on a body is by observing that it moves as a free particle along the geodesics of spacetime. But how, without already knowing the geodesics or the projective structure of spacetime is it possible to determine which particles are free and which are not? And to determine the projective structure of spacetime it is necessary to use free particles. 