The Law of Inertia plays a key role in the scheme of constructive axioms for the General Theory of Relativity. A new formulation of this law which avoids the circularity problems inherent in previous formulations is presented. The empirical status of this law and the manner in which it provides a non-conventional foundation for the Law of Motion and the definition of physical forces is established. First, quite general path structures are discussed which are not defined at the outset in (...) terms of geodesic paths and which require for their description only the local differential topological structure. Secondly, a number of theorems are presented which serve as purely local differential topological criteria for singling out from among the general path structures the geodesic ones that represent the inertial structure of spacetime. (shrink)