  While Wittgenstein and Russell seem to have held that the constituents of atomic facts are to be determined on the basis of a priori considerations, Armstrong (1997, 2004) advocates an a posteriori form of logical atomism. On his view, atomic facts are composed of particulars and simple universals (properties and relations). The latter are objective features of the world that ground the objective resemblances between particulars and explain their causal powers. Accordingly, what particulars and universals there are will have to be determined on the basis of total science.