  Asymptotically flat static causal fermion systems are introduced. Their total
mass is defined as a limit of surface layer integrals which compare the
measures describing the asymptotically flat spacetime and a vacuum spacetime
near spatial infinity. Our definition does not involve any regularity
assumptions; it even applies to singular or generalized "quantum" spacetimes. A
positive mass theorem is proven. Our methods and results explain why and how
the causal action principle incorporates the nonlinear effects of gravity for
static systems.
