  of objects corresponding respectively to the notions of events,  particle paths and light rays, which are taken as  primitive. The set \(M\) of events is assumed to have a Hausdorff  topology with a countable basis in order to state local axioms  through the use of such terms as “neighborhood”. Members  of the sets \(\mathcal{P} = \{P, Q, P_{1}, Q_{1}, \ldots \}\) and  \(\mathcal{L} = \{ L, N, L_{1}, N_{1}, \ldots \}\) are subsets of  \(M\) that represent the possible or actual paths of massive  particles and light rays in spacetime. 